CN114498584B - Virtual impedance-based fault current suppression method for multi-drop point hybrid cascade direct current system - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H—ELECTRICITY
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Abstract
The invention discloses a virtual impedance-based multi-drop point hybrid cascade direct current system fault current suppression method, which comprises the steps of introducing virtual impedance control into MMC control, providing a virtual impedance calculation method and introducing a virtual impedance adjusting coefficient, and when detecting that the difference value between an actually measured current value and a rated current value exceeds a set threshold value, sending a trigger signal 0 to enable the value of the adjusting coefficient to be the output value of a virtual impedance control module, and putting the virtual impedance into use; otherwise, by sending a trigger signal 1, switching the value of the adjusting coefficient to be 0, the virtual impedance is not put into use any more, and the control of all the MMCs is changed into the control in normal operation; and after the fault crossing time is finished, the system recovers to operate. The invention can be realized only by changing the control structure of the VSC, and an additional current limiting device is not required to be installed, so that the economy is greatly improved; the control link can be directly put into operation by triggering during the fault, and the power transmission of the hybrid high-voltage direct-current system is not required to be temporarily stopped.
Description
Technical Field
The invention relates to the technical field of hybrid direct-current power transmission, in particular to a fault current suppression method of a multi-drop hybrid cascade direct-current system based on virtual impedance.
Background
Hybrid direct current transmission has become an important development direction of direct current transmission technology in recent years due to the combination of the respective advantages of conventional direct current (LCC-HVDC) and flexible direct current (VSC-HVDC). The rectifier station of the receiving-end cascade hybrid direct-current transmission (as shown in figure 1) is formed by connecting 2 groups of 12-pulse LCCs in series, the inverter station is formed by connecting 1 group of 12-pulse commutation converters (LCCs) and Voltage Source Converters (VSCs) in parallel in series, the low-end VSCs are expanded into a plurality of VSCs which are connected in parallel and are located in different regional power grids, the transmission power of the hybrid direct-current system is increased, and meanwhile, a multi-drop-point structure is also beneficial to the staged construction of the engineering (the MMC in figure 1 is a modular multilevel converter and belongs to one type of VSC). And because the receiving end converter station is cascaded with the LCC converter station after the plurality of VSC converter stations are connected in parallel, a multi-end system is actually formed, and the power distribution capability of the plurality of VSC converter stations is given.
When a receiving end alternating current system has a short-circuit fault, although the hybrid cascade direct current system has the technical advantages of both the LCC direct current and the VSC direct current, the phase commutation failure is still an inevitable problem due to the existence of the LCC converter on the inversion side. And no matter power surplus or LCC commutation failure can lead to VSC overcurrent, overvoltage and arouse VSC protection action, cause VSC valves shutting, can lead to power transmission interrupt even.
The document [1] (Liu Zeng flood, Wang Shaowu, seed sesame, Huangyong, Guxiansha, Shokuh, Liu fir, Li Ying, Zhao Wen is strong.) the controllable self-recovery energy dissipation device [ J ] suitable for the mixed cascade ultra-high voltage DC transmission system, China Motor engineering bulletin, 2021,41(02): 514-.
In the document [2] (permit winter, li nay, mei niai, etc.. transient current suppression method [ J ]. global energy internet 2020, 3(2): 166-.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a method for suppressing a fault current in a multi-drop hybrid cascade dc system based on a virtual impedance, wherein the method is used to suppress a cascade hybrid dc/ac fault current by introducing the virtual impedance into MMC group control. The technical scheme is as follows:
a multi-drop point hybrid cascade direct current system fault current suppression method based on virtual impedance comprises the following steps:
step 1: introducing virtual impedance control in MMC control
MMC adopts double-ring control, and in an MMC station adopting constant-power outer ring control, the direct current of the MMC and the direct voltage are connected through the current inner ring control, so thatIntroduction of virtual impedance Zv _ \ in inner loop control MMC ；
Dq axis component i of reference current to be output by outer loop control ref,d And i ref,q As an input signal for inner loop current control; and taking into account the coupling term omega b L T i d 、ω b L T i q And u sd 、u sq Influence on the dq-axis current component, where i d And i q Is the common junction dq axis current component, L T Is an equivalent inductance, omega, at the AC side of the MMC b Is the reference frequency, u, on the AC side sd 、u sq A dq axis voltage component for the point of common connection; output voltage u of current inner loop controller cref,d And u cref,q Obtaining the equivalent output voltage u of the AC side of the converter after the PWM modulation process cd ，u cq (ii) a By introducing a virtual impedance Z before PWM modulation v_MMC To regulate and control the output voltage u cref,d And u cref,q To a new voltage u' cref,d And u' cref,q ；
Step 2: calculating a virtual impedance
Deducing an impedance model of the MMC to obtain an output impedance value of the MMC, wherein the output impedance Z of the constant power station equivalent controlled current source EP The derivation process is as follows:
the inner loop output quantity in dq coordinate system is expressed as
Where ω is the angular frequency of the AC system, G c ，G LT Is a matrix code, Δ u cref,d And Δ u cref,q The disturbance amount is the output voltage dq axis component of the current inner loop controller; k c (s) is the transfer function of the current inner loop PI controller; delta i ref,d And Δ i ref,q A disturbance amount of a dq-axis component of a reference current output for an outer loop control; delta i d And Δ i q A disturbance amount that is a current component of the dq axis of the common connection point; Δ u sd And Δ u sq Of voltage components of the dq axis of the common connection pointThe disturbance amount; Δ u sd 、Δu sq Represented by the formula:
wherein Z is T Is the equivalent impedance of the alternating current side; r T Is an equivalent resistor at the AC side; Δ u cd And Δ u cq Disturbance quantity of an equivalent output voltage dq axis component at the AC side of the MMC converter station;
the modulation process of the PWM link is expressed by a delay link as follows:
wherein, T sw For time delay of switching, f sw Is the switching frequency; g PWM S is a Laplace operator which is a transfer function of the delay link;
voltage reference value u of current inner loop output cref,d And u cref,q Obtaining the equivalent output voltage u of the AC side of the converter after the PWM modulation process cd ，u cq Is composed of
Combined vertical type (1) -is shown in formula (4)
Current reference value i ref,d And i ref,q Is expressed as
Wherein, G u0 And G i0 Is a matrix code, K p (s) for power outer loop PI controllersA transfer function; u. u sd0 And u sq0 A steady state value of the voltage component of the dq axis for the point of common connection; i all right angle d0 And i q0 Is the steady state value of the dq axis current component;
combined vertical type (2), formula (5), formula (6)
Wherein E is an identity matrix; g iA And G uA Is a matrix code;
eliminating AC-side electrical disturbance Δ u cd 、Δu cq 、Δi d And Δ i q Obtaining:
wherein m is d0 And m q0 Modulation degrees, G, of d-and q-axes, respectively A Is a matrix code; v. of dc0 And i dc0 Respectively obtaining steady-state values of direct-current side voltage and current of the MMC converter station; Δ i dc And Δ v dc Disturbance quantities of direct-current side voltage and current of the MMC converter station are respectively; u. of cd0 And u cq0 And (3) steady-state value of the equivalent output voltage dq axis component on the AC side of the MMC converter station.
Output impedance Z of equivalent controlled current source of constant power station EP Is finally expressed as
Z EP Obtaining the output impedance Z of the MMC constant power station after further equivalence P Is expressed as
Wherein, C eq Is an equivalent capacitance, R arm And L arm Are respectively provided withBridge arm resistance and bridge arm inductance;
after introducing the dummy impedance, the formula (5) is rewritten into
Wherein, K VIC Adjusting the coefficient for the virtual impedance;
further, equation (8) is rewritten as:
wherein G is V Is a matrix code;
the system impedance after introducing the virtual impedance is obtained according to the formula (12)
From equation (13), after the virtual impedance controller is put into use, the equivalent impedance of the system is partially increased on the basis of the original model, and the increased amount is the virtual impedance value, and the value is:
wherein G is V By adjusting the introduced virtual impedance adjustment coefficient K VIC Is changed to thereby change the virtual impedance Z v_MMC The value of (d);
and step 3: implementing virtual impedance control
When the actual measured current value I of the direct current side is detected dc_m And a DC side rated current value I dc_rated When the difference exceeds the set threshold, the adjustment coefficient K is adjusted by sending a trigger signal 0 VIC The value of (A) is the output value of the virtual impedance control module, and the virtual impedance is put into use when the actual measured current value I of the direct current side is detected dc_m And a DC side rated current value I dc_rated When the difference value of (A) is not set to the threshold value, the adjustment coefficient K is switched by sending a trigger signal 1 VIC The value of (3) is 0, the virtual impedance is not put into use any more, and the control of all the MMCs is changed into the control in normal operation;
and 4, step 4: and after the fault ride-through time of 200-300 ms, the system recovers to operate.
Further, the fault current suppression method is also suitable for controlling the MMC station by using the constant direct current voltage.
The invention has the beneficial effects that:
1) compared with the method described in the document 1, the method can be realized only by changing the control structure of the VSC, and an additional current limiting device is not required to be installed, so that the economy is greatly improved;
2) compared with the method described in document 2, the method can be directly put into use by triggering the control link in case of failure, and the power transmission of the hybrid high-voltage direct-current system does not need to be stopped temporarily.
Drawings
Fig. 1 is a schematic circuit diagram of a multi-drop receiving-end cascade hybrid direct-current system.
Fig. 2 is a control block diagram of the inverter side constant power control MMC introducing virtual impedance.
FIG. 3 is an MMC single-phase equivalent model.
Fig. 4 is a block diagram of a virtual impedance trigger control.
FIG. 5 is a comparison graph of equivalent impedance of a constant power control MMC before and after adding a virtual impedance.
FIG. 6 is a graph comparing bridge arm current of MMC3 before and after the addition of a virtual impedance in example 1; (a) no virtual impedance control, and (b) virtual impedance control.
FIG. 7 is a graph comparing bridge arm current of MMC3 before and after the addition of a virtual impedance in example 2; (a) no virtual impedance control, and (b) virtual impedance control.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments. The invention can inhibit the cascade type mixed direct current and alternating current fault current by the technology of introducing the virtual impedance in the MMC group control, and the concrete mode is as follows:
(1) introducing virtual impedance control in MMC control:
the MMC adopts double-ring control, and an MMC station adopting constant-power outer ring control is taken as an example. The MMC direct current and the direct voltage are connected through the inner loop control, so that the virtual impedance Z is considered to be introduced into the inner loop control v_MMC The MMC control block diagram after introducing the virtual impedance control is shown in fig. 2. In the figure and hereinafter, the variable subscripts g, s and c represent variables at the outlet of the alternating current side of the alternating current system, the Point of Common Coupling (PCC) and the MMC converter station, respectively, the subscripts d and q represent components of the d axis and the q axis, respectively, the subscript abc represents a variable in a three-phase coordinate system, the subscript dc represents a variable on the direct current side, the subscript ref represents a reference value, and the subscript' represents a variable after introducing the virtual impedance control; wherein L is T And R T Respectively, the equivalent rated inductance and resistance on the AC side, L g An inductance of an alternating current system; omega b Is an AC side reference frequency; k p (s) and K c (s) are the transfer functions of the power outer loop and current inner loop PI controllers, respectively. i.e. i d And i q I of the outer loop output, d-and q-axis current components, respectively ref,d And i ref,q As an input signal to the inner loop current controller; the phase-locked loop PLL provides an alternating voltage signal phase angle theta for the coordinate conversion process. Omega b Is the reference frequency on the AC side, taking into account the coupling term omega b L T i d 、ω b L T i q And u sd 、u sq Influence on the dq-axis current component, output u of the current inner loop cref,d And u cref,q After the PWM modulation process, the equivalent output voltage u of the AC side of the converter can be obtained cd ，u cq . The PWM link is represented by a delay link, T sw For time delay of switching, f sw Is the switching frequency.
Taking an MMC station adopting constant-power outer-loop control as an example, the direct current of the MMC and the direct current voltage are connected through the inner-loop control of the current, so that the virtual impedance Zv \uis introduced into the inner-loop control MMC (ii) a Dq axis component i of reference current to be output by outer loop control ref,d And i ref,q As inner loop current controlA modulated input signal; and taking into account the coupling term omega b L T i d 、ω b L T i q And u sd 、u sq Influence on the dq-axis current component, where i d And i q D-and q-axis current components, omega, respectively b Is the reference frequency on the ac side; output voltage u of current inner loop controller cref,d And u cref,q Obtaining the equivalent output voltage u of the AC side of the converter after the PWM modulation process cd ，u cq (ii) a By introducing a virtual impedance Z before PWM modulation v_MMC To regulate and control the output voltage u cref,d And u cref,q To new voltage u' cref,d And u' cref,q 。
(2) The value of the virtual impedance is as follows:
in order to realize the effect of virtual impedance control in step 1, it is essential to obtain the virtual impedance Z v_MMC The value of (c). Calculating Z v_MMC The value of (3) needs to derive the impedance model of the MMC to obtain the output impedance value of the MMC. The single-phase equivalent circuit of MMC at fundamental frequency is shown in FIG. 3, wherein SM n As MMC sub-module, R arm And L arm Respectively, bridge arm resistance and bridge arm inductance, C eq Is an equivalent capacitance, Z EP And Z EV Output impedance, Z, of Equivalent Controlled Current Sources (ECCS) corresponding to a constant power station and a constant voltage station, respectively P And Z V Corresponding to the output impedances of the constant power station and the constant voltage station, respectively. Z P And Z V The derivation process is similar, as follows for the derivation of Z P For example.
Z EP The specific derivation process of (2) is as follows:
the inner loop output quantity in dq coordinate system can be expressed as
Where ω is the angular frequency of the AC system, G c ，G LT The amount of band Δ in this text is representative of its perturbation amount, denoted by the matrix index. Δ u sd 、Δu sd Can be represented by the following formula
Wherein, Z T Is the equivalent impedance of the AC side.
The modulation process of the PWM link can be represented by a delay link as
Wherein, T sw For time delay of switching, f sw Is the switching frequency.
Voltage reference u of current inner loop output cref,d And u cref,q After the PWM modulation process, the equivalent output voltage u of the AC side of the converter can be obtained cd ，u cq Is composed of
Combined vertical type (1) -formula (4) is
Current reference value i ref,d And i ref,q Is expressed as
Wherein G is u0 ，G i0 For matrix codes, the subscript "0" herein refers to the value at the steady state point with the electrical quantity.
Combined vertical type (2), formula (5), formula (6) get
Wherein G is u0 ，G i0 Is the matrix code number, E is the identity matrix.
Eliminating AC-side electrical disturbance Δ u cd 、Δu cq 、Δi d And Δ i q Can obtain the product
Wherein m is d0 And m q0 Modulation degrees, G, of d-and q-axes, respectively A Is the matrix code.
Z EP Is finally expressed as
Z EP Obtaining the output impedance Z of the MMC constant power station after further equivalence P Is expressed as
According to the MMC control block diagram after introducing the virtual impedance in FIG. 2, the formula (5) can be rewritten into
Wherein K VIC The value of the variable introduced in the virtual impedance control.
Whereby the formula (8) can be rewritten as
Wherein G V Is a matrix code.
The system impedance after introducing the dummy impedance can be obtained according to equation (12) as
From equation (13), after the virtual impedance controller is used, the equivalent impedance of the system is partially increased based on the original model, and the increased amount is the virtual impedance value, which is:
wherein G is V Can be adjusted by adjusting the introduced virtual impedance parameter K VIC Is changed so that the virtual impedance Z can be changed v_MMC The numerical value of (c).
It can be seen from the above that, after the virtual impedance is introduced, the equivalent impedance of the system is increased on the original basis. The mechanism of limiting the fault current by introducing the virtual impedance in the control of the MMC group is to reduce the fault current by increasing the system impedance value, so as to achieve the purpose of current limiting. Since the high-frequency impedance has a large influence on the fault current during the fault, it can be seen from fig. 5 that the high-frequency impedance of the system is significantly increased after the introduction of the virtual impedance, thereby achieving the purpose of current limiting.
(3) The implementation process of the virtual impedance control comprises the following steps:
introduced virtual impedance adjustment coefficient K VIC The purpose is to adjust the magnitude of the virtual impedance value. I is dc_m As a measure of the direct side current, I dc_rated Δ I being the rated value of the direct side current dc The difference between the measured value and the nominal value. The virtual impedance is only put into use when a fault occurs, and the virtual impedance is not put into use under the normal working condition. When the system works normally, the trigger module generates a trigger signal '1', and K at the moment VIC Is 0, the equivalent virtual impedance is not injected; when the actual measured current value exceeds the rated current value of 0.9pu, the trigger module generates a trigger signal '0', K VIC The value of (d) is the output value of the control module, and the virtual impedance is applied at this time.
(4) When the actual measured current value is detected not to exceed the rated current value of 0.9pu, the adjustment coefficient K is switched VIC Is a normal value of 0, i.e., control of all MMCs becomes control at normal operation;
(5) and after the fault ride-through time of 200-300 ms, the system recovers to operate.
Example (b):
calculation and verification were performed using a cascade type mixed direct current as an example as shown in fig. 1.
Example 1: the inverter side LCC alternating current bus is provided with a single-phase short circuit grounding fault scene, the starting time of the fault is 3s, and the duration time of the fault is 0.1 s. Fig. 6 is a graph showing a comparison of bridge arm current values flowing through MMC3 before and after the virtual impedance control is put into operation when the inverter-side LCC ac bus is short-circuited.
Example 2: and a single-phase short circuit grounding fault is set on the MMC alternating-current bus at the inversion side, the starting time of the fault is 3s, and the duration time is 0.1 s. Fig. 7 is a graph showing a comparison of bridge arm current values flowing through MMC3 before and after the virtual impedance control is put into operation when the inverter-side LCC ac bus is short-circuited.
It can be seen that the fault current of the MMC is reduced to within twice the normal operating current, i.e., the safe operation region. The MMC is effectively prevented from being locked, power can be normally transmitted, the hybrid direct current can effectively pass through when the alternating current bus on the inversion side fails, and the fact that the proposed strategy is effective is proved.
Claims (2)
1. A fault current suppression method of a multi-drop point hybrid cascade direct current system based on virtual impedance is characterized by comprising the following steps:
step 1: introducing virtual impedance control in MMC control
The MMC adopts double-loop control, and in an MMC station adopting constant-power outer loop control, the direct current and the direct current voltage of the MMC are connected through the current inner loop control, so that virtual impedance Zv _ \ MMC ；
Dq axis component i of reference current to be output by outer loop control ref,d And i ref,q As an input signal for inner loop current control; and taking into account the coupling term omega b L T i d 、ω b L T i q And u sd 、u sq Influence on the dq-axis current component, where i d And i q Is the dq-axis current component of the common junction, L T Is an MMC AC side equivalent inductor, omega b Is the reference frequency, u, on the AC side sd 、u sq A dq axis voltage component for the point of common connection; output voltage u of current inner loop controller cref,d And u cref,q Obtaining the equivalent output voltage u of the AC side of the converter after the PWM modulation process cd ，u cq (ii) a By introducing a virtual impedance Z before PWM modulation v_MMC To regulate and control the output voltage u cref,d And u cref,q To a new voltage u' cref,d And u' cref,q ；
Step 2: calculating a virtual impedance
Deducing an impedance model of the MMC to obtain an output impedance value of the MMC, wherein the output impedance Z of the constant power station equivalent controlled current source EP The derivation process is as follows:
the inner loop output in dq coordinate system is expressed as
Where ω is the angular frequency of the AC system, G c ，G LT Is a matrix code, Δ u cref,d And Δ u cref,q The disturbance amount is the output voltage dq axis component of the current inner loop controller; k c (s) is the transfer function of the current inner loop PI controller; Δ i ref,d And Δ i ref,q A disturbance amount of a dq-axis component of a reference current output for the outer loop control; Δ i d And Δ i q A disturbance amount that is a common connection point dq-axis current component; Δ u sd And Δ u sq A disturbance amount that is a common connection point dq-axis voltage component; Δ u sd 、Δu sq Represented by the formula:
wherein Z is T Is the equivalent impedance of the alternating current side; r is T Is an equivalent resistor at the AC side; Δ u cd And Δ u cq Disturbance quantity of an equivalent output voltage dq axis component at the AC side of the MMC converter station;
the modulation process of the PWM link is expressed by a delay link as follows:
wherein, T sw For time delay of switching, f sw Is the switching frequency; g PWM For the delay link transfer function, s is a laplacian operator;
voltage reference u of current inner loop output cref,d And u cref,q Obtaining the equivalent output voltage u of the AC side of the converter after the PWM modulation process cd ，u cq Is composed of
Combined vertical type (1) -is shown in formula (4)
Current reference value i ref,d And i ref,q Is expressed as
Wherein G is u0 And G i0 Is a matrix code, K p (s) is the transfer function of the power outer loop PI controller; u. of sd0 And u sq0 A steady state value of the dq axis voltage component for the point of common connection; i all right angle d0 And i q0 Is the steady state value of the dq axis current component;
combined vertical type (2), formula (5), formula (6) get
Wherein E is an identity matrix; g iA And G uA Is a matrix code;
eliminating AC-side electrical disturbance Δ u cd 、Δu cq 、Δi d And Δ i q Obtaining:
wherein m is d0 And m q0 Degree of modulation, G, of d-and q-axes, respectively A Is a matrix code; v. of dc0 And i dc0 Respectively obtaining steady-state values of direct-current side voltage and current of the MMC converter station; Δ i dc And Δ v dc Disturbance quantities of direct-current side voltage and current of the MMC converter station are respectively; u. of cd0 And u cq0 The steady state value of the equivalent output voltage dq axis component at the AC side of the MMC converter station;
output impedance Z of equivalent controlled current source of constant power station EP Is finally expressed as
Z EP Obtaining the output impedance Z of the MMC constant power station after further equivalence P Is expressed as
Wherein, C eq Is an equivalent capacitance, R arm And L arm Respectively a bridge arm resistance and a bridge arm inductance;
after introducing the dummy impedance, the formula (5) is rewritten into
Wherein, K VIC Adjusting the coefficient for the virtual impedance;
further, equation (8) is rewritten as:
wherein, G V Is a matrix code;
the system impedance after introducing the virtual impedance is obtained according to the formula (12)
From equation (13), after the virtual impedance controller is used, the equivalent impedance of the system is partially increased based on the original model, and the increased amount is the virtual impedance value, which is:
wherein G is V By adjusting the introduced virtual impedance adjustment coefficient K VIC Is changed to thereby change the virtual impedance Z v_MMC The value of (d);
and 3, step 3: implementing virtual impedance control
When the actual measured current value I of the direct current side is detected dc_m And a DC side rated current value I dc_rated When the difference exceeds the set threshold, the adjustment coefficient K is adjusted by sending a trigger signal 0 VIC The value of (A) is the output value of the virtual impedance control module, and the virtual impedance is put into use when the actual measured current value I of the direct current side is detected dc_m And a DC side rated current value I dc_rated When the difference value of (A) is not set to the threshold value, the adjustment coefficient K is switched by sending a trigger signal 1 VIC The value of (3) is 0, the virtual impedance is not put into use any more, and the control of all the MMCs is changed into the control in normal operation;
and 4, step 4: and after the fault crossing time of 200-300 ms, the system recovers to operate.
2. The virtual impedance-based multi-drop hybrid cascaded direct current system fault current suppression method according to claim 1, wherein the fault current suppression method is also applicable to a constant direct current voltage controlled MMC station.
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| CN113067358A (en) * | 2021-03-25 | 2021-07-02 | 中国矿业大学 | Method for reducing direct current side current-limiting reactor of MMC type direct current network |
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| CN105634258A (en) * | 2015-01-30 | 2016-06-01 | 华北电力大学 | Virtual impedance-based AC side fault current suppression method for MMC |
| WO2018040466A1 (en) * | 2016-08-27 | 2018-03-08 | 许继集团有限公司 | Direct-current fault ride-through method and computer storage medium |
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