A flexible DC passive control system and method

Abstract

The present invention relates to a flexible DC passive control system and method. In the proposed flexible DC passive control method, in the no-load operation mode, the voltage control function directly controls the AC voltage in a closed loop, the system is fully controlled, and has good stability. During the AC system fault ride-through process, the current limiting function limits the positive sequence current to within the limit value, the negative sequence current suppression function suppresses the negative sequence current on the transformer valve side under asymmetric fault conditions, and the inductance voltage drop of the voltage control function can speed up the AC voltage recovery speed after the fault is cleared.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current power transmission, and in particular to a flexible direct current passive control system and method. Background Art

[0002] Flexible DC has the advantages of no commutation failure, independent control of active power and reactive power, small harmonics, power supply to passive systems, and easy construction of multi-terminal systems. It has been applied to various scenarios such as high-voltage DC transmission, DC distribution, energy routers, and offshore wind power transmission. The passive operation condition of flexible DC refers to the flexible DC converter not being connected to the AC grid. Specifically, it may include flexible DC power supply to passive loads, flexible DC connected to island grids, and flexible DC connected to wind farms. Flexible DC is not connected to the AC grid, which poses new challenges to the stability and fault recovery performance of the system.

[0003] Existing flexible DC passive control methods include Figure 6 As shown, the voltage control function outputs the d-axis current reference value Idref and the q-axis current reference value Iqref according to the d-axis component Ud and the q-axis component Uq of the transformer grid-side voltage through the adjustment of the PI link. The current control function outputs the reference voltages Ucdref and Ucqref according to the d-axis component Id, the q-axis component Iq, and Idref and Iqref of the transformer valve-side current through the adjustment of the PI link. After the dq / abc coordinate transformation, the fundamental component of the modulation voltage is output.

[0004] The existing flexible DC passive control scheme has two defects. Defect 1: In the flexible DC no-load operation mode (i.e. I d =0,I q =0), when the system starts, the PI link of voltage control and current control will inevitably enter a restricted state, and the system is actually in an uncontrollable state. Defect 2: Voltage control relies on the PI link for adjustment, and the voltage response speed is slow. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a flexible DC passive control system and method. In no-load operation mode, the system is fully controlled and has good stability; in the AC system fault recovery process, the voltage response speed is fast.

[0006] To achieve the above object, the present invention provides a flexible DC passive control system, including a voltage extraction module, a current extraction module, a positive sequence voltage control module, a negative sequence current suppression module, a conversion module, a positive sequence current limiting module and a current limiting module;

[0007] The voltage extraction module collects the three-phase voltage on the grid side of the transformer, extracts the positive and negative sequence, and calculates the d-axis component U of the positive sequence voltage on the grid side.dpos , q-axis component U qpos ;

[0008] The current extraction module collects the three-phase current on the valve side, extracts the positive and negative sequence, and calculates the d-axis component I of the positive sequence current on the valve side. dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg ;

[0009] The positive sequence voltage control module adjusts U dpos Close to 1 and get the positive sequence voltage d-axis reference value U cdposref ; Adjust U through the second PI link qpos Close to 0, the positive sequence voltage q axis reference value U cqposref ; If overcurrent is detected, reset the first PI link and the second PI link;

[0010] The negative sequence current suppression module adjusts I dneg Close to 0 and get the negative sequence voltage d-axis reference value U cdnegref ; Adjust I through the fourth PI link qneg Close to 0 and get the negative sequence voltage q axis reference value U cqnegref ; After the asymmetric fault is detected, the third PI link and the fourth PI link are reset;

[0011] The conversion module converts the d-axis and q-axis positive sequence voltage reference values ​​U cdposref And negative sequence voltage reference value U cdnegref Perform coordinate transformation to obtain the positive and negative sequence voltage reference values ​​of the three-phase stationary coordinate system and superimpose them to obtain the fundamental wave component of the three-phase modulation voltage, calculate the modulation wave and send it to the valve control;

[0012] The positive sequence current limiting module limits the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude is within the range of the upper current limit and the lower current limit;

[0013] The current limiting module calculates the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude of the positive sequence current d-axis component I dpos , q-axis component I qpos The upper and lower current limits.

[0014] Furthermore, the positive sequence voltage control module adjusts U dpos Close to 1, the output of the first PI link is equal to sL*I dposSum, then sum the value with wL*I qpos Subtract and get the positive sequence voltage d-axis reference value U cdposref ; The second PI link adjusts U qpos Close to 0, the output of the second PI link is equal to sL*I qpos Sum, then add wL*I dpos , get the positive sequence voltage q axis reference value U cqposref ; s is the Laplace operator, L is the combined inductance of the bridge arm reactor and the transformer leakage reactance, and w is the angular frequency of the three-phase voltage on the grid side of the transformer.

[0015] Furthermore, the negative sequence current suppression module adjusts I dneg Close to 0, the output of the third PI link is multiplied by -1 and then equals wL*I qneg Sum and get the negative sequence voltage d-axis reference value U cdnegref ; Adjust I through the fourth PI link qneg Close to 0, the output of the fourth PI link is multiplied by -1 and then equals wL*I dneg Subtract and get the negative sequence voltage q axis reference value U cqnegref .

[0016] Furthermore, the positive sequence current limiting module controls the valve side positive sequence current d-axis component I dpos When the current I dpos Greater than the current limit value I dmax When the control current I dpos Equal to the current limit value I dmax ; Current after limiting I dpos with I dpos The difference is adjusted to be close to 0 through the fifth PI link, and the output of the fifth PI link is limited; the output value of the fifth PI link is multiplied by -1 and (wL*I qpos +U dpos ) and obtain the positive sequence current d-axis limit value U dlim ; For the valve side positive sequence current q-axis component I qpos When the current I qpos Greater than the current limit value I qmax When the control current I qpos Equal to the current limit value I qmax ; Current after limiting I qpos with I qpos The difference is adjusted to be close to 0 through the sixth PI link, and the output of the sixth PI link is limited; the output value of the sixth PI link is multiplied by -1 and (U qpos -wL*I dpos ) and obtain the positive sequence current q-axis limit value U qlim; After the overcurrent is detected, the fifth PI link and the sixth PI link are reset.

[0017] Furthermore, the current limiting module controls the d-axis component and q-axis component I of the valve side positive sequence current. qpos After smoothing through the first-order inertia link, the amplitude is calculated to obtain the current amplitude; the maximum current limit value I max Multiply it by the smoothed positive sequence current d-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current d-axis maximum limit value I dmax , the minimum limit value of the valve side positive sequence current d axis I dmin =-1*I dmax ; Maximum current limit value I max Multiply it by the smoothed positive sequence current q-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current q-axis maximum limit value I qmax , the minimum limit value of the valve side positive sequence current q axis I qmin =-1*I qmax .

[0018] Furthermore, it also includes an overcurrent detection module, which calculates the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude of the current is determined to determine whether it exceeds the maximum current limit value I max If exceeded, the output high level signal will be widened and delayed to obtain an overcurrent signal.

[0019] Furthermore, the invention also includes an asymmetric fault end detection module, which converts the negative sequence voltage q axis reference value U cqnegref Multiply by the valve side negative sequence current q-axis component I qneg , the product is compared with 0. If it is less than 0, the output high-level signal is delayed to obtain an asymmetric fault end signal.

[0020] Another aspect of the present invention provides a flexible DC passive control method, comprising:

[0021] Collect the three-phase voltage on the grid side and the three-phase current on the valve side of the transformer, extract the positive and negative sequence of the three-phase voltage on the grid side and the three-phase current on the valve side, and calculate the d-axis component U of the positive sequence voltage on the grid side. dpos , q-axis component U qpos , the d-axis component of the positive sequence current on the valve side I dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg ;

[0022] Adjust U through the first PI link dpos Close to 1 and get the positive sequence voltage d-axis reference value U cdposref; Adjust U through the second PI link qpos Close to 0, the positive sequence voltage q axis reference value U cqposref ; If overcurrent is detected, reset the first PI link and the second PI link;

[0023] Adjust I through the third PI link dneg Close to 0 and get the negative sequence voltage d-axis reference value U cdnegref ; Adjust I through the fourth PI link qneg Close to 0 and get the negative sequence voltage q axis reference value U cqnegref ; After the asymmetric fault is detected, the third PI link and the fourth PI link are reset;

[0024] Limit the positive sequence current d-axis component I on the valve side dpos , q-axis component I qpos The amplitude is within the range of the upper current limit and the lower current limit;

[0025] Calculate the d-axis component of the positive sequence current on the valve side I dpos , q-axis component I qpos The amplitude of the positive sequence current d-axis component I dpos , q-axis component I qpos The upper and lower current limits;

[0026] The d-axis and q-axis positive sequence voltage reference values ​​U cdposref And negative sequence voltage reference value U cdnegref Coordinate transformation is performed to obtain the positive-sequence and negative-sequence voltage reference values ​​of the three-phase stationary coordinate system and superimpose them, and the modulation wave is calculated and sent to the valve control.

[0027] Furthermore, U is adjusted through the first PI link dpos Close to 1 and get the positive sequence voltage d-axis reference value U cdposref , including calculations of:

[0028]

[0029] Where, s is the Laplace operator, L is the combined inductance of the bridge arm reactor and the transformer leakage reactance, w is the angular frequency of the three-phase voltage on the grid side of the transformer, K pu is the proportional gain, T nu is the integration time constant; OCL=0 indicates that no overcurrent is detected; when OCL=1, it indicates that overcurrent is detected and the integrator is cleared, U dlim is the d-axis voltage limit value.

[0030] Furthermore, U is adjusted through the second PI link qpos Close to 0, the positive sequence voltage q axis reference value U cqposref , including calculations of:

[0031]

[0032] Among them, U qlim is the q-axis voltage limit value.

[0033] Further, the third PI link is used to adjust I dneg Close to 0, the output of the third PI link is multiplied by -1 and then equals wL*I qneg Sum and get the negative sequence voltage d-axis reference value U cdnegref , including calculations of:

[0034]

[0035] Among them, K pi is the proportional gain; T ni is the integration time constant.

[0036] Further, the fourth PI link adjusts I qneg Close to 0 and get the negative sequence voltage q axis reference value U cqnegref , including calculations of:

[0037]

[0038] Furthermore, the d-axis component of the positive sequence current on the valve side is limited dpos , q-axis component I qp The amplitude of os includes the following within the current upper and lower limits: the positive sequence current d-axis component I dpos Limiting, with I dpos The difference is adjusted to be close to 0 through the fifth PI link, and the output of the fifth PI link is limited. The output value of the fifth PI link is multiplied by -1 and (wL*I qpos +U dpos ) and obtain the positive sequence current d-axis limit value U dlim ; For the valve side positive sequence current q-axis component I qpos Limiting, with I qpos The difference is adjusted to be close to 0 through the sixth PI link, and the output of the sixth PI link is limited; the output value of the sixth PI link is multiplied by -1 and (U qpos -wL*I dpos ) and obtain the positive sequence current q-axis limit value U qlim ; After the overcurrent is detected, the fifth PI link and the sixth PI link are reset.

[0039] Furthermore, the d-axis component and q-axis component of the valve-side positive sequence current I qpos After smoothing through the first-order inertia link, the amplitude is calculated to obtain the current amplitude; the maximum current limit value I maxMultiply it by the smoothed positive sequence current d-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current d-axis maximum limit value I dmax , the minimum limit value of the valve side positive sequence current d axis I dmin =-1*I dmax ; Maximum current limit value I max Multiply it by the smoothed positive sequence current q-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current q-axis maximum limit value I qmax , the minimum limit value of the valve side positive sequence current q axis I qmin =-1*I qmax .

[0040] The above technical solution of the present invention has the following beneficial technical effects:

[0041] (1) In the flexible DC passive control method proposed in the present invention, the voltage control function directly controls the AC voltage in a closed loop under no-load operation mode, and the system is fully controlled and has good stability. During the AC system fault ride-through process, the current limiting function limits the positive sequence current to within the limit value, and the negative sequence current suppression function suppresses the negative sequence current on the transformer valve side under asymmetric fault conditions. The inductance voltage drop of the voltage control function can speed up the recovery of the AC voltage after the fault is cleared.

[0042] (2) The flexible DC positive sequence current limiting function proposed in the present invention takes effect when the current exceeds the limit, preventing the converter from overcurrent during the AC system fault ride-through, resulting in protection action. dpos and I qpos After the amplitude limiting process, it is used as the reference value of the d-axis and q-axis current, and adjusted by PI so that I dpos and I qpos The difference from the conventional flexible DC inner loop current control is that when the converter overcurrent is detected, the PI link of the positive sequence current limiting function is released and reset, and the positive sequence current limiting function starts to work. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a topological diagram of a flexible DC converter;

[0044] Figure 2 The flexible DC passive control block diagram provided by the present invention;

[0045] Figure 3 A block diagram of the flexible DC positive sequence current limitation provided by the present invention;

[0046] Figure 4 A functional block diagram of the fault detection provided by the present invention;

[0047] Figure 5 This is a flexible DC passive control flow chart of the present invention;

[0048] Figure 6 This is the existing flexible DC passive control block diagram. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0050] Figure 1 This is a topological diagram of the flexible DC converter (the present invention selects a voltage source converter VSC) provided by the present invention. Each bridge arm of the VSC is composed of N submodules (SM) connected in series with bridge arm reactors. The flexible DC passive operating condition means that the VSC is not connected to the large AC power grid. In scenarios such as flexible DC supplying power to loads, flexible DC connecting to island power grids, and flexible DC connecting to wind farms, VSC needs to apply passive control methods.

[0051] Figure 2 The passive control system of the present invention realizes the functions of positive sequence voltage control, negative sequence current suppression, positive sequence current limitation, overcurrent detection and asymmetric fault termination detection.

[0052] The flexible DC passive control system comprises a voltage extraction module, a current extraction module, a positive sequence voltage control module, a negative sequence current suppression module, a conversion module, a positive sequence current limiting module, a current limiting module, an overcurrent detection module and an asymmetric fault end detection module.

[0053] The voltage extraction module collects the three-phase voltage on the grid side of the transformer, extracts the positive and negative sequence, and calculates the d-axis component U of the positive sequence voltage on the grid side. dpos , q-axis component U qpos .

[0054] The current extraction module collects the three-phase current on the valve side, extracts the positive and negative sequence, and calculates the d-axis component I of the positive sequence current on the valve side. dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg .

[0055] The positive sequence voltage control module adjusts U dpos Close to 1 and get the positive sequence voltage d-axis reference value U cdposref ; Adjust U through the second PI link qpos Close to 0, the positive sequence voltage q axis reference value U cqposref; If overcurrent is detected, the first PI link and the second PI link are reset. Positive sequence voltage control can be divided into d-axis voltage control and q-axis voltage control. U dpos =1, U qpos Equal to 0. qpos *wL and I dpos *wL is a decoupling term, which reduces the coupling effect between the positive sequence d-axis current and the positive sequence q-axis current. dpos *sL and I qpos *sL is the inductor voltage drop. Introducing the inductor voltage drop can speed up the voltage control response. In the no-load operation mode, the positive sequence voltage control function does not go through the current control link. dpos and U qpos The positive sequence voltage reference value can be adjusted quickly. s is the Laplace operator, L is the comprehensive inductance of the bridge arm reactor and the transformer leakage reactance, and w is the transformer grid side voltage angular frequency.

[0056] The negative sequence current suppression module adjusts I dneg Close to 0 and get the negative sequence voltage d-axis reference value U cdnegref ; Adjust I through the fourth PI link qneg Close to 0 and get the negative sequence voltage q axis reference value U cqnegref ; After the asymmetric fault is detected, the third PI link and the fourth PI link are reset; the negative sequence current suppression can be divided into d-axis current suppression and q-axis current suppression, and the PI link is used to control I dneg and I qneg are all equal to 0, I dneg *wL and I qneg *wL is a decoupling term, which reduces the coupling effect between the negative sequence d-axis current and the negative sequence q-axis current. The positive sequence current limiting function takes effect when the current exceeds the limit, and the output d-axis voltage limit value U dlim and q-axis voltage limit value U qlim The overcurrent detection function outputs a current limit-exceeding signal OCL when the current exceeds the limit. When OCL is at a high level, the positive-sequence component control of the modulation voltage switches from positive-sequence voltage control to positive-sequence current limiting, and the integrators of the positive-sequence voltage control of the first PI link and the second PI link are cleared.

[0057] The conversion module converts the d-axis and q-axis positive sequence voltage reference values ​​U cdposref And negative sequence voltage reference value U cdnegrefCoordinate transformation is performed to obtain the positive and negative sequence voltage reference values ​​of the three-phase stationary coordinate system and superimpose them to obtain the three-phase modulation voltage fundamental component output power supply. The d-axis and q-axis positive sequence voltage reference values ​​and negative sequence voltage reference values ​​are converted into the positive and negative sequence voltage reference values ​​of the three-phase coordinate system through dq / abc coordinate transformation. The superposition of the two is the three-phase modulation voltage fundamental component, and the modulation wave is calculated, and then the modulation wave is sent to the valve control to generate a trigger pulse. θ is the positive sequence voltage phase angle, and -θ is the negative sequence voltage phase angle. θ is used to convert from a two-phase rotating coordinate system (i.e., dq coordinate system) to a three-phase stationary coordinate system (i.e., abc coordinate system).

[0058] The positive sequence current limiting module limits the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude is within the current upper limit and current lower limit range; the current limiting module calculates the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude of the positive sequence current d-axis component I dpos , q-axis component I qpos The upper and lower current limits. Figure 3 This is a block diagram of the flexible DC positive sequence current limitation provided by the present invention. dpos and I qpos For limiting processing, the maximum limit value is I dmax and I qmax , the minimum limit value is I dmin and I qmin Through PI regulation, I dpos and I qpos Within the limit value. When there is no fault, the PI link input is 0; wL*I qpos and wL*I dpos It is a decoupling term, which reduces the coupling effect between the d-axis current and the q-axis current. dpos and U qpos is the voltage feedforward term. When the overcurrent detection function outputs OCL = 1, the PI link of the positive sequence current limiting function is released and reset, and the positive sequence current limiting function begins to work. The current limiting function will I dpos and I qpos After smoothing through the first-order inertia link, the amplitude is calculated to obtain the current amplitude. max Multiply by the smoothed positive sequence current d-axis component, and then divide by the current amplitude to obtain the d-axis maximum limit value I dmax , d-axis minimum limit value I dmin =-1*I dmax . Maximum current limit value I max Multiply by the smoothed positive sequence current q-axis component, and then divide by the current amplitude to obtain the q-axis maximum limit value Iqmax , q-axis minimum limit value I qmin =-1*I qmax .

[0059] The asymmetric fault detection module is based on the q-axis negative sequence voltage reference value U cqnegref and q-axis negative sequence current I qneg It is determined whether the asymmetric fault is ended, and an asymmetric fault end signal Fend is output. When Fend=1, the integrator of the negative sequence current suppression is cleared.

[0060] Figure 4 : is a functional block diagram of the fault detection provided by the present invention. Fault detection includes overcurrent detection and asymmetric fault termination detection. The overcurrent detection function is implemented by I dpos and I qpos The calculation of the valve side current amplitude, the current amplitude and the maximum allowable current limit I max For comparison, after the widening delay link, the overcurrent signal OCL is output. The asymmetric fault end detection function calculates the q-axis negative sequence voltage reference value U cqnegref and q-axis negative sequence current I qneg The product is compared with 0, and is processed by the delay link to determine whether the asymmetric fault is over, and an asymmetric fault end signal Fend is output.

[0061] like Figure 4 As shown, the current amplitude is related to I max After passing through the comparator, the overcurrent status signal is output, which has only two states: 0 and 1. 1 indicates overcurrent status, and 0 indicates non-overcurrent status. During the fault process, the current oscillates, and the duration of the overcurrent status signal (state 1) may be relatively short. The overcurrent status signal needs to be converted into a high-level signal with a longer duration, that is, widening and delay processing. When a fault occurs in the AC system, the current amplitude is greater than I max , output over-current signal.

[0062] Negative sequence voltage q axis reference value U cqnegref Multiply by the valve side negative sequence current q-axis component I qneg , the product is compared with 0, and a status signal is output. During the process of fault termination, the status signal output by the comparator may switch between 0 and 1 multiple times, and a delay process is required. That is, when the duration of the output 1 state reaches the threshold, it can be confirmed that the fault has ended.

[0063] Another aspect of the present invention provides a flexible DC passive control method, combining Figure 5 The following steps are involved:

[0064] Step S100: Collect the three-phase voltage on the grid side of the transformer and the three-phase current on the valve side, extract the positive and negative sequence of the voltage and current, and calculate the grid side positive sequence voltage d-axis component Udpos , q-axis component U qpos , the d-axis component of the positive sequence current on the valve side I dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg .

[0065] Step S200: positive sequence voltage control function design, the specific steps include: dpos The difference is calculated through the PI link, and the calculated value is equal to sL*I dpos Sum, then sum the value with wL*I qpos Subtract and get the positive sequence voltage d-axis reference value U cdposref . qpos The difference is calculated through the PI link, and the calculated value is equal to sL*I qpos Sum, then add wL*I to the sum dpos , get the positive sequence voltage q axis reference value U cqposref After overcurrent is detected (i.e. OCL = 1), the PI link is reset. s is the Laplace operator, L is the combined inductance of the bridge arm reactor and the transformer leakage reactance, and w is the angular frequency of the transformer grid-side voltage.

[0066] The control system collects the three-phase voltage on the grid side of the transformer and the three-phase current on the valve side. The system frequency is set to the rated frequency. The positive and negative sequence of the voltage and current are extracted to calculate the positive-sequence voltage d-axis component U on the grid side. dpos , q-axis component U qpos , the d-axis component of the positive sequence current on the valve side I dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg Positive sequence voltage control can be divided into d-axis voltage control and q-axis voltage control. dpos =1, U qpos Equal to 0. The d-axis voltage control equation is:

[0067]

[0068] The q-axis voltage control equation is:

[0069]

[0070] In the d-axis and q-axis voltage control equations, I qpos *wL and I dpos *wL is a decoupling term, which reduces the coupling effect between the positive sequence d-axis current and the positive sequence q-axis current. dpos *sL and I qpos*sL is the inductor voltage drop. Introducing the inductor voltage drop can speed up the voltage control response speed. Different from the existing voltage control, the voltage control proposed in the present invention does not go through the current control link in the no-load operation mode, and can be based on U dpos and U qpos Rapidly adjust the positive sequence voltage reference value. s is the Laplace operator, L is the combined inductance of the bridge arm reactor and the transformer leakage reactance, and w is the transformer grid side voltage angular frequency. OCL is the current limit signal. When OCL=1, the positive sequence voltage control switches to the current limit control output Udlim and Uqlim, and the integrator of the positive sequence voltage control is cleared.

[0071] Step S300: negative sequence current suppression function design, the specific steps include: dneg The difference is calculated through the PI link, and the calculated value is multiplied by -1 and then added to wL*I qneg Sum and get the negative sequence voltage d-axis reference value U cdnegref . 0 and I qneg The difference is calculated through the PI link, and the calculated value is multiplied by -1 and then added to wL*I dneg Subtract and get the negative sequence voltage q axis reference value U cqnegref After the asymmetric fault is detected (i.e., Fend = 1), the PI link is reset. L is the combined inductance of the bridge arm reactor and the transformer leakage reactance, and w is the grid-side voltage angular frequency of the transformer.

[0072] The negative sequence current suppression proposed in the present invention can be divided into d-axis negative sequence current control and q-axis negative sequence current control, and controls Idneg = 0, Iqneg = 0. The d-axis negative sequence current control equation is:

[0073]

[0074] The q-axis negative sequence current control equation is:

[0075]

[0076] I dneg *wL and I qneg *wL is a decoupling term that reduces the coupling effect between the negative-sequence d-axis current and the negative-sequence q-axis current.

[0077] Step S400: positive sequence current limiting function design, the specific steps include: dpos For limiting processing, the maximum limit value is I dmax , the minimum limit value is I dmin . The limit processing value is related to I dpos The difference is calculated through the PI link, and the calculated value is multiplied by -1 and then added to wL*I qpos +U dposSum and get the positive sequence current d-axis limit value U dlim . Will I qpos For limiting processing, the maximum limit value is I qmax , the minimum limit value is I qmin . The limit processing value is related to I qpos The difference is calculated through the PI link, and the calculated value is multiplied by -1 and then added to U qpos -wL*I dpos Sum and get the positive sequence current q axis limit value U qlim After the overcurrent is detected (i.e. OCL = 0), the PI link is reset. L is the combined inductance of the bridge arm reactor and the transformer leakage reactance, and w is the grid-side voltage angular frequency of the transformer.

[0078] Step S500: Current limiting function design, the specific steps include: dpos and I qpos After smoothing through the first-order inertia link, the amplitude is calculated to obtain the current amplitude. max Multiply by the smoothed positive sequence current d-axis component, and then divide by the current amplitude to obtain the d-axis maximum limit value I dmax , d-axis minimum limit value I dmin =-1*I dmax . Maximum current limit value I max Multiply by the smoothed positive sequence current q-axis component, and then divide by the current amplitude to obtain the q-axis maximum limit value I qmax , q-axis minimum limit value I qmin =-1*I qmax .

[0079] Step S600: designing the overcurrent detection function, the specific steps include: dpos and I qpos Amplitude calculation is performed to obtain the current amplitude, which is equal to the maximum limit value I max The comparison is performed, and the comparison output signal is widened and delayed to obtain the overcurrent signal OCL.

[0080] Step S700: Design of asymmetric fault end detection function, the specific steps include: negative sequence voltage q axis reference value U cqnegref Multiply by I qneg , the product is compared with 0, and the comparison output signal is delayed to obtain the asymmetric fault end signal Fend.

[0081] The difference from the traditional flexible DC negative sequence suppression is that after the asymmetric fault ends, the integrators of the d-axis and q-axis negative sequence current control are reset. The asymmetric fault end detection criterion is:

[0082] U cqnegref *Iqneg <0.

[0083] Step S800: modulating voltage synthesis, the specific steps include: cdposref , U dlim Input to the two-out-of-one selector, whose control signal is OCL, to obtain the d-axis positive sequence component of the modulation voltage. cqposref , U qlim Input to the two-out-of-one selector, whose control signal is OCL, to obtain the q-axis positive sequence component of the modulation voltage. Perform dq / abc coordinate transformation on the d-axis and q-axis positive and negative components of the modulation voltage to obtain the three-phase modulation voltage positive sequence component. cdnegref and U cqnegref Perform dq / abc coordinate transformation to obtain the negative sequence component of the three-phase modulated voltage. The positive sequence component and the negative sequence component of the three-phase modulated voltage are summed to obtain the fundamental wave component of the three-phase modulated voltage.

[0084] In summary, the present invention relates to a flexible DC passive control system and method. In the proposed flexible DC passive control method, the voltage control function directly performs closed-loop control on the AC voltage under no-load operation mode, and the system is fully controlled and has good stability. During the AC system fault crossing process, the current limiting function limits the positive sequence current to within the limit value, the negative sequence current suppression function suppresses the negative sequence current on the transformer valve side under asymmetric fault conditions, and the inductance voltage drop of the voltage control function can speed up the recovery speed of the AC voltage after the fault is cleared.

[0085] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, and do not constitute a limitation of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalent forms of such scope and boundaries.

Claims

1. A flexible DC passive control system, characterized in that: It includes a voltage extraction module, a current extraction module, a positive sequence voltage control module, a negative sequence current suppression module, a conversion module, a positive sequence current limiting module and a current limiting module; The voltage extraction module collects the three-phase voltage on the grid side of the transformer, extracts the positive and negative sequence, and calculates the d-axis component U of the positive sequence voltage on the grid side. dpos , q-axis component U qpos ; The current extraction module collects the three-phase current on the valve side, extracts the positive and negative sequence, and calculates the d-axis component I of the positive sequence current on the valve side. dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg ; The positive sequence voltage control module adjusts U dpos Close to 1, the output of the first PI link is equal to sL*I dpos Sum, then sum the value with wL*I qpos Subtract and get the positive sequence voltage d-axis reference value U cdposref , including calculations of: Adjust U through the second PI link qpos Close to 0, the output of the second PI link is equal to sL*I qpos Sum, then add wL*I dpos , get the positive sequence voltage q axis reference value U cqposref , including calculations of: If overcurrent is detected, the first PI link and the second PI link are reset; s is the Laplace operator, L is the comprehensive inductance of the bridge arm reactor and the transformer leakage reactance, w is the angular frequency of the three-phase voltage on the transformer grid side, K pu is the proportional gain, T nu is the integration time constant; OCL=0 indicates that no overcurrent is detected; when OCL=1, it indicates that overcurrent is detected and the integrator is cleared, U dlim is the d-axis voltage limit value, U qlim is the q-axis voltage limit value; The negative sequence current suppression module adjusts I dneg Close to 0, the output of the third PI link is multiplied by -1 and then equals wL*I qneg Sum and get the negative sequence voltage d-axis reference value U cdnegref ; Adjust I through the fourth PI link qneg Close to 0, the output of the fourth PI link is multiplied by -1 and then equals wL*I dneg Subtract and get the negative sequence voltage q axis reference value U cqnegref ; After the asymmetric fault is detected, the third PI link and the fourth PI link are reset; The conversion module converts the d-axis and q-axis positive sequence voltage reference values ​​U cdposref And negative sequence voltage reference value U cdnegref Perform coordinate transformation to obtain the positive and negative sequence voltage reference values ​​of the three-phase stationary coordinate system and superimpose them to obtain the fundamental wave component of the three-phase modulation voltage, calculate the modulation wave and send it to the valve control; The positive sequence current limiting module limits the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude is within the range of the upper current limit and the lower current limit; The current limiting module calculates the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude of the positive sequence current d-axis component I dpos , q-axis component I qpos The upper and lower current limits.

2. The flexible DC passive control system according to claim 1, characterized in that: The positive sequence current limiting module controls the positive sequence current d-axis component I dpos When the current I dpos Greater than the current limit value I dmax When the control current I dpos Equal to the current limit value I dmax ; Current after limiting I dpos with I dpos The difference is adjusted to be close to 0 through the fifth PI link, and the output of the fifth PI link is limited; The output value of the fifth PI link is multiplied by -1 and then (wL*I qpos +U dpos ) and obtain the positive sequence current d-axis limit value U dlim ; For the valve side positive sequence current q-axis component I qpos When the current I qpos Greater than the current limit value I qmax When the control current I qpos Equal to the current limit value I qmax ; Current after limiting I qpos with I qpos The difference is adjusted to be close to 0 through the sixth PI link, and the output of the sixth PI link is limited; The output value of the sixth PI link is multiplied by -1 and then added to (U qpos -wL*I dpos ) and obtain the positive sequence current q-axis limit value U qlim ; After the overcurrent is detected, the fifth PI link and the sixth PI link are reset.

3. The flexible DC passive control system according to claim 2, characterized in that: The current limiting module controls the d-axis component and q-axis component I of the positive sequence current on the valve side. qpos After being smoothed by the first-order inertia link, the amplitude is calculated to obtain the current amplitude; Current maximum limit value I max Multiply it by the smoothed positive sequence current d-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current d-axis maximum limit value I dmax , the minimum limit value of the valve side positive sequence current d axis I dmin =-1*I dmax ; Current maximum limit value I max Multiply it by the smoothed positive sequence current q-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current q-axis maximum limit value I qmax , the minimum limit value of the valve side positive sequence current q axis I qmin =-1*I qmax .

4. The flexible DC passive control system according to claim 3, characterized in that: Also includes, an overcurrent detection module, calculates the valve side positive sequence current d-axis component I dpos , q-axis component I qpos The amplitude of the current is determined to determine whether it exceeds the maximum current limit value I max If exceeded, the output high level signal will be widened and delayed to obtain an overcurrent signal.

5. The flexible DC passive control system according to claim 4, characterized in that: Also included is an asymmetric fault end detection module, which converts the negative sequence voltage q axis reference value U cqnegref Multiply by the valve side negative sequence current q-axis component I qneg , the product is compared with 0. If it is less than 0, the output high-level signal is delayed to obtain an asymmetric fault end signal.

6. A flexible DC passive control method, characterized in that: include: Collect the three-phase voltage on the grid side and the three-phase current on the valve side of the transformer, extract the positive and negative sequence of the three-phase voltage on the grid side and the three-phase current on the valve side, and calculate the d-axis component U of the positive sequence voltage on the grid side. dpos , q-axis component U qpos , the d-axis component of the positive sequence current on the valve side I dpos , q-axis component I qpos , the d-axis component of the negative sequence current on the valve side I dneg , q-axis component I qneg ; Adjust U through the first PI link dpos Close to 1, the output of the first PI link is equal to sL*I dpos Sum, then sum the value with wL*I qpos Subtract and get the positive sequence voltage d-axis reference value U cdposref , including calculations of: Adjust U through the second PI link qpos Close to 0, the output of the second PI link is equal to sL*I qpos Sum, then add wL*I dpos , get the positive sequence voltage q axis reference value U cqposref , including calculations of: If overcurrent is detected, the first PI link and the second PI link are reset; s is the Laplace operator, L is the comprehensive inductance of the bridge arm reactor and the transformer leakage reactance, w is the angular frequency of the three-phase voltage on the transformer grid side, K pu is the proportional gain, T nu is the integration time constant; OCL=0 indicates that no overcurrent is detected; when OCL=1, it indicates that overcurrent is detected and the integrator is cleared, U dlim is the d-axis voltage limit value, U qlim is the q-axis voltage limit value; Adjust I through the third PI link dneg Close to 0, the output of the third PI link is multiplied by -1 and then equals wL*I qneg Sum and get the negative sequence voltage d-axis reference value U cdnegref ; Adjust I through the fourth PI link qneg Close to 0, the output of the fourth PI link is multiplied by -1 and then equals wL*I dneg Subtract and get the negative sequence voltage q axis reference value U cqnegref ; After the asymmetric fault is detected, the third PI link and the fourth PI link are reset; Limit the positive sequence current d-axis component I on the valve side dpos , q-axis component I qpos The amplitude is within the range of the upper current limit and the lower current limit; Calculate the d-axis component of the positive sequence current on the valve side I dpos , q-axis component I qpos The amplitude of the positive sequence current d-axis component I dpos , q-axis component I qpos The upper and lower current limits; The d-axis and q-axis positive sequence voltage reference values ​​U cdposref And negative sequence voltage reference value U cdnegref Coordinate transformation is performed to obtain the positive-sequence and negative-sequence voltage reference values ​​of the three-phase stationary coordinate system and superimpose them, and the modulation wave is calculated and sent to the valve control.

7. The flexible DC passive control method according to claim 6, characterized in that: Adjust I through the third PI link dneg Close to 0, the output of the third PI link is multiplied by -1 and then equals wL*I qneg Sum and get the negative sequence voltage d-axis reference value U cdnegref , including calculations of: Among them, K pi is the proportional gain; T ni is the integration time constant.

8. The flexible DC passive control method according to claim 7, characterized in that: Adjust I through the fourth PI link qneg Close to 0 and get the negative sequence voltage q axis reference value U cqnegref , including calculations of:

9. The flexible DC passive control method according to claim 8, characterized in that: Limit the positive sequence current d-axis component I on the valve side dpos , q-axis component I qpos The amplitude of the current upper limit and the current lower limit includes: the positive sequence current d-axis component I dpos Limiting, with I dpos The difference is adjusted to be close to 0 through the fifth PI link, and the output of the fifth PI link is limited. The output value of the fifth PI link is multiplied by -1 and (wL*I qpos +U dpos ) and obtain the positive sequence current d-axis limit value U dlim ; For the valve side positive sequence current q-axis component I qpos Limiting, with I qpos The difference is adjusted to be close to 0 through the sixth PI link, and the output of the sixth PI link is limited; the output value of the sixth PI link is multiplied by -1 and (U qpos -wL*I dpos ) and obtain the positive sequence current q-axis limit value U qlim ; After the overcurrent is detected, the fifth PI link and the sixth PI link are reset.

10. The flexible DC passive control method according to claim 9, characterized in that: The d-axis component and q-axis component of the positive sequence current on the valve side I qpos After being smoothed by the first-order inertia link, the amplitude is calculated to obtain the current amplitude; Current maximum limit value I max Multiply it by the smoothed positive sequence current d-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current d-axis maximum limit value I dmax , the minimum limit value of the valve side positive sequence current d axis I dmin =-1*I dmax ; Current maximum limit value I max Multiply it by the smoothed positive sequence current q-axis component, and then divide it by the current amplitude to obtain the valve side positive sequence current q-axis maximum limit value I qmax , the minimum limit value of the valve side positive sequence current q axis I qmin =-1*I qmax .

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