MMC sub-module synchronous pre-charging method based on power balance

Abstract

The invention relates to the technical field of power systems, in particular to an MMC sub-module synchronous pre-charging method based on power balance. Charging power is obtained by designing closed-loop control of sub-module capacitors on the basis of a power balance principle, then total power released by an alternating current side or a direct current side is calculated through power conservation and charging current feedback control, synchronous charging of capacitor voltages of upper and lower bridge arm sub-modules is completed, power difference is introduced, and synchronous charging of the capacitor voltages of the upper and lower bridge arm sub-modules is completed. And voltage sharing of the sub-modules in the bridge arms and voltage sharing between the bridge arms are realized. The method can be applied to alternating-current side and direct-current side charging, and due to the fact that synchronous charging of the upper bridge arm sub-module and the lower bridge arm sub-module is achieved, the pre-charging speed is remarkably increased. In addition, the same voltage and current double-closed-loop control is adopted in the pre-charging state and the normal state of the MMC, so that the starting time can be obviously shortened. According to the invention, a novel control strategy is provided for the pre-charging of the MMC sub-module, and the method has certain significance in reducing the speed increase of fault restart and normal start in high-voltage direct-current power transmission.

Description

Technical Field

[0001] The invention relates to the technical field of power systems, and in particular to a MMC submodule synchronous pre-charging method based on power balance. Background Art

[0002] Modular Multilevel Converter (MMC) plays a key role in power electronics technology, especially in high-voltage direct current transmission and energy storage systems. When restarting after a fault or starting normally, the submodule capacitor charges too slowly, which will cause energy waste, grid instability, increase operating costs and limit the application of fast energy conversion. Its fast charging capability has a significant impact on grid regulation, renewable energy grid connection, improving energy conversion efficiency and equipment miniaturization. Fast charging can not only adapt to the unstable output of renewable energy and ensure grid stability, but also reduce energy loss and improve energy utilization. In addition, with the increase in charging speed, the size and weight of power electronic equipment can be reduced, reducing costs, which is of great significance to the improvement of response speed, system stability and transmission efficiency of direct current transmission.

[0003] In terms of the open-loop control strategy for MMC pre-charging:

[0004] 1. The literature [A.Lesnicar and R.Marquardt, An innovative modular multilevel converter topology suitable for a wide power range 2003IEEE Bologna PowerTech Conference Proceedings. (2003)] proposed a separate auxiliary DC power supply to provide additional power to pre-charge the submodule capacitors. However, it has insufficient advantages in terms of charging cycle, cost, flexibility, etc.

[0005] 2. The literature [J.Xu, et al. New Precharge and Submodule Capacitor Voltage Balancing Topologies of Modular Multilevel Converter for VSC-HVDC Application. IEEE PES Asia-Pacific Power and Energy Engineering Conference IEEE, (2011)] proposed a combined circuit using an auxiliary power supply to reduce the charging time. Despite this, the cost cannot be reduced and the control is relatively complicated. Due to the characteristics of the open-loop strategy, these methods all have the limitation that the charging current is uncontrollable. Therefore, the charging current is completely dependent on the circuit parameters, and if there is an unexpected disturbance or parameter change, it may cause an overcurrent problem.

[0006] 3. The literature [Li B., et al. Closed-Loop Precharge Control of Modular Multilevel Converters During Start-Up Processes. IEEE Transactions on Power Electronics 30.2: 524-531 (2015)] proposed a closed-loop method to control the startup of MMC from the AC side main voltage, and gradually increase the voltage to the rated value through a cascade structure. In addition, the literature [Shi, X., Liu, B., Wang, Z., Li, Y., et al. Modelling, control design and analysis of a startup scheme for modular multilevel converters. IEEE Transactions on Industrial Electronics, 62 (11), 7009-7024. (2015)] proposed another closed-loop startup control method and pointed out that the resonance between the bridge arm inductance and the submodule capacitor has certain restrictions on the charging time.

[0007] However, due to the asynchronous charging of the upper and lower bridge arm submodule capacitors, the existing closed-loop control strategy still cannot maximize the pre-charging speed. In addition, since the control strategy is tuned at the end of the pre-charging process, the startup time is further increased.

[0008] Therefore, in order to improve the sub-module capacitor voltage charging speed before the MMC restarts after a fault or starts normally, and to improve the fault ride-through capability of the MMC, it is necessary to design a control method that can realize the synchronous charging of the upper and lower bridge arm sub-module capacitors, that is, a MMC sub-module synchronous pre-charging method based on power balance in the present application. Summary of the invention

[0009] The purpose of the present invention is to propose a MMC submodule synchronous precharging method based on power balance to address the problem that due to the asynchronous charging of the upper and lower bridge arm submodule capacitors in the background technology, the existing closed-loop control strategy still cannot maximize the precharging speed and increases the startup time.

[0010] The technical solution of the present invention is a MMC submodule synchronous pre-charging method based on power balance, which is applied to AC side or DC side charging scenarios, including:

[0011] After uncontrolled charging to the limit value using the MMC's own performance, the required power of each phase is calculated by comparing the difference between the actual value and the required value of the MMC submodule voltage, and the reference voltage value of each phase is obtained through the bridge arm voltage algorithm to achieve synchronous charging of the upper and lower bridge arms;

[0012] Calculate the power difference between the upper and lower bridge arms, and obtain the amplitude representing the fundamental frequency circulating current by comparing the difference between the sum of the average values ​​of the upper and lower bridge arm submodule voltages through the PI controller;

[0013] The difference between the base frequency circulating current described in the instruction and the actual circulating current is passed through the PR controller, and the physical quantity output by the controller represents the voltage drop on the bridge arm reactor. The voltage difference between the upper and lower bridge arms is compensated to achieve the voltage equalization control of the upper and lower bridge arms.

[0014] Calculate the power difference of each submodule in the bridge arm, take the average value of the instantaneous value of the capacitor voltage of the submodule in the bridge arm as the command voltage value, multiply the difference between the command voltage value and the actual voltage value by the bridge arm current, and then pass through the proportional controller to obtain the additional reference voltage value of each submodule;

[0015] Adding the additional reference voltage value to the output voltage reference value of the submodule to obtain an independent reference voltage value for each submodule, thereby achieving capacitor voltage balance of each submodule;

[0016] After the pre-charging is completed, the normal operation state control is directly entered, and the reference current value is modified from the calculated value during the charging process to the rated value, without the need to design a control strategy and switch it separately.

[0017] Optionally, during synchronous pre-charging control on the MMC AC side, the released power of the AC grid is equal to the sum of the charging powers of all three-phase submodules, which is expressed as:

[0018] P ac=P asm_sum +P bsm_sum +P bsm_sum ;

[0019] Where: P ac Represents the power released by the grid, P bsm_sum Represents the total charging power of all submodules in phase B, P csm_sum Represents the total charging power of all sub-modules in phase C.

[0020] Optionally, the output power on the AC side of the MMC is reflected in the AC side current, that is;

[0021]

[0022] E represents the grid phase voltage amplitude, and I represents the grid phase current amplitude.

[0023] Optionally, the upper and lower bridge arm voltage reference values ​​U are obtained by the bridge arm voltage algorithm. xp and U xn , expressed as:

[0024]

[0025] U xp Indicates the upper bridge arm voltage, U xn Represents the voltage of the lower bridge arm, where x = a, b, c, representing the three phases ABC; U cref Indicates the submodule capacitor voltage command value; U x It represents the three-phase differential mode voltage reference value, which is obtained through grid-connected current loop control.

[0026] Optionally, in order to ensure that the voltage difference between the upper and lower bridge arm capacitors is 0, a bridge arm power difference control algorithm is used. The bridge arm power difference expression is:

[0027] ΔP a =-2e a i diffa ;

[0028] Where: i diffa represents the A-phase bridge arm circulation, e a Represents the phase voltage amplitude of phase A.

[0029] Optionally, the voltage difference between the upper and lower bridge arm submodules is set to 0, and the feedback value is the sum of the average value of the capacitor voltage of the upper bridge arm submodule (U apc1(avg) +…+U apcN(avg) ) and the sum of the average voltage of the capacitor of the lower bridge arm submodule (U anc1(avg) +…+U ancN(avg) ) is passed through the PI controller, and the output physical quantity represents the amplitude of the fundamental frequency circulating current I diffa1, the phase of the base frequency circulating current is selected to be in phase with the grid voltage, and then the command base frequency circulating current i diffa1_ref The actual circulation diffa The difference passes through the PR controller, and the physical quantity output represents the voltage drop U on the bridge arm reactor. diffa , the final bridge arm voltage expression is:

[0030]

[0031] U diffx Indicates the reference voltage value on the bridge arm reactance.

[0032] Optionally, during the synchronous pre-charging control of the MMC DC side, the release power of the DC grid is equal to the sum of the charging powers of all three-phase submodules, which is expressed as:

[0033] P dc =P asm_sum +P bsm_sum +P bsm_sum ;

[0034] Where: P dc Represents the power released on the DC side, P asm_sum Represents the total charging power of all submodules in phase A, P bsm_sum Represents the total charging power of all submodules in phase B, P csm_sum Represents the total charging power of all sub-modules in phase C.

[0035] Optionally, the output power on the DC side of the MMC is reflected in the DC side current, that is, the DC side output power P dc It is expressed as;

[0036] P dc =U dc i dc ;

[0037] P dc Indicates the DC side output power, U dc Indicates the DC side output voltage, i dc Indicates the DC side output current;

[0038] Since the three-phase structure of MMC is symmetrical, the DC side current is evenly divided among the three-phase bridge arms, expressed as:

[0039]

[0040] Where: i diffa0 、i diffb0 、i diffc0 They represent the DC components in the current of the A-phase bridge arm, the B-phase bridge arm, and the C-phase bridge arm respectively.

[0041] Optionally, the upper and lower bridge arm voltage reference values ​​U are obtained by the bridge arm voltage algorithm. xp and U xn The bridge arm voltage expression is:

[0042]

[0043] Optionally, the independent reference voltage values ​​of the i-th submodule of the upper and lower bridge arms are expressed as:

[0044]

[0045] U xpsmi_ref and U xnsmi_ref Respectively represent the independent reference voltage values ​​of the i-th submodule of the upper and lower bridge arms; K p Indicates the control coefficient; U xpci and U xnci Respectively represent the actual voltage values ​​of the upper and lower bridge arms of the ith submodule; i xp and i xn Represent the upper and lower bridge arm currents respectively.

[0046] Compared with the prior art, the present invention has the following beneficial technical effects:

[0047] 1. The present invention can be applied to charging on the AC side and the DC side. Since the synchronous charging of the upper and lower bridge arm submodules is realized, the pre-charging speed is significantly improved.

[0048] 2. Since the same voltage and current dual closed-loop control is used in both the MMC pre-charging and normal states, the startup time can be significantly shortened;

[0049] 3. The present invention provides a new control strategy for precharging the MMC submodule, which is of certain significance for reducing the speed of fault restart and normal startup of high-voltage direct current transmission;

[0050] 4. According to the requirements of charging current and charging time, the upper and lower bridge arms can be charged asynchronously and synchronously to meet engineering needs. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is the system power control block diagram when the MMC AC side is started;

[0052] Figure 2 This is the block diagram of the voltage difference control between the upper and lower bridge arms of the MMC;

[0053] Figure 3 This is the block diagram of the voltage equalization control in the MMC bridge arm;

[0054] Figure 4 This is the system power control block diagram when the MMC DC side is started. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0056] A method for synchronous precharging of MMC submodules based on power balance. When the MMC restarts or starts normally after a fault, the submodules need to be accurately precharged to speed up the startup speed. Therefore, it is necessary to first calculate the required charging power and distribute it to each submodule capacitor, and then control the voltage balancing of multiple submodule capacitors in the bridge arm, and then balance the voltage of the upper and lower bridge arms and the voltage balancing of the phase-to-phase bridge arms. There are two different control strategies for synchronous precharging of the MMC AC side and synchronous precharging of the MMC DC side.

[0057] Example 1, the synchronous pre-charging control strategy for the MMC AC side is as follows:

[0058] like Figure 1 As shown in the figure, according to the power conservation law, the sum of the charging power of all three-phase submodules is equal to the release power of the AC power grid, that is:

[0059] P ac =P asm_sum +P bsm_sum +P bsm_sum ;

[0060] Where: P ac Represents the power released by the grid, P bsm_sum Represents the total charging power of all submodules in phase B, P csm_sum Represents the total charging power of all sub-modules in phase C.

[0061] The output power on the AC side is reflected in the AC side current, that is:

[0062]

[0063] Where: E represents the grid phase voltage amplitude, I represents the grid phase current amplitude.

[0064] The bridge arm voltage algorithm using the process of dq change, decoupling, and dq inverse transformation is used to obtain the upper and lower bridge arm voltage reference values ​​U xp and U xn The bridge arm voltage reference value is expressed as:

[0065]

[0066] Where: U xp Indicates the upper bridge arm voltage, U xn Represents the voltage of the lower bridge arm, where x = a, b, c, representing the three phases ABC; U cref Indicates the submodule capacitor voltage command value; U xIndicates the three-phase differential mode voltage reference value.

[0067] like Figure 2 As shown, in order to ensure that the voltage difference between the upper and lower bridge arm capacitors is 0 during the controllable charging process, a bridge arm power difference control algorithm is introduced.

[0068] Taking phase A as an example, the bridge arm power difference expression is:

[0069] ΔP a =-2e a i diffa ;

[0070] Where: i diffa represents the A-phase bridge arm circulation, e a Represents the phase voltage amplitude of phase A.

[0071] The voltage difference between the upper and lower bridge arm submodules is set to 0. The voltage difference here is the ideal value of the voltage difference between the upper and lower bridge arm submodules. It is preset to 0 during control, and the actual value is continuously approached to 0 through the control algorithm. The feedback value is the sum of the average value of the capacitor voltage of the upper bridge arm submodule (U apc1(avg) +…+U apcN(avg )) and the sum of the average voltage of the capacitor of the lower bridge arm submodule (U anc1(avg) +…+U ancN(avg) ) is passed through the PI controller, and the output physical quantity represents the amplitude of the fundamental frequency circulating current I diffa1 , the phase of the base frequency circulating current is selected to be in phase with the grid voltage, and then the command base frequency circulating current i diffa1_ref The actual circulation diffa The difference passes through the PR controller, and the physical quantity output represents the voltage drop U on the bridge arm reactor. diffa .

[0072] The final bridge arm voltage expression is:

[0073]

[0074] Where: U diffx Indicates the reference voltage value on the bridge arm reactance.

[0075] like Figure 3 As shown, first, the average value of the instantaneous value of the capacitor voltage of the submodule in the bridge arm is used as the command voltage value, the command voltage value is subtracted from the actual voltage value and multiplied by the bridge arm current, and then the additional reference voltage value of each submodule is obtained through the proportional controller. The additional voltage value is added to the output voltage reference value of the submodule. The output voltage reference value of the submodule is the reference voltage value of each phase divided by the number of submodules in each phase bridge arm, and finally an independent reference voltage value of each submodule is obtained.

[0076] The independent reference voltage values ​​of the i-th submodule of the upper and lower bridge arms are as follows:

[0077]

[0078] Where: U xpsmi_ref and U xnsmi_ref Respectively represent the independent reference voltage values ​​of the i-th submodule of the upper and lower bridge arms; K p Indicates the control coefficient; U xpci and U xnci Respectively represent the actual voltage values ​​of the upper and lower bridge arms of the ith submodule; i xp and i xn Represent the upper and lower bridge arm currents respectively.

[0079] At this point, the synchronous charging and voltage balancing control of the capacitors on the AC side of the upper and lower bridge arm submodules of the MMC are completed. After that, the normal operation state can be directly entered without the need to design a control strategy and switch separately.

[0080] Example 2, the synchronous pre-charging control strategy for the MMC DC side is as follows:

[0081] According to the power conservation law, the sum of the charging power of all three-phase submodules is equal to the release power on the DC side, that is,

[0082] P dc =P asm_sum +P bsm_sum +P bsm_sum ;

[0083] Where: P dc Represents the power released on the DC side, P asm_sum Represents the total charging power of all submodules in phase A, P bsm_sum Represents the total charging power of all submodules in phase B, P csm_sum Represents the total charging power of all sub-modules in phase C.

[0084] The output power on the DC side is reflected in the DC current, that is:

[0085] P dc =U dc i dc ;

[0086] Where: P dc Indicates the DC side output power, U dc Indicates the DC side output voltage, i dc Indicates the DC side output current;

[0087] Since the three-phase structure of the system is symmetrical, the DC side current is evenly divided among the three-phase bridge arms, that is:

[0088]

[0089] Where: i diffa0 、idiffb0 、i diffc0 They represent the DC components in the current of the A-phase bridge arm, the B-phase bridge arm, and the C-phase bridge arm respectively.

[0090] System power control during DC side startup is as follows Figure 4 As shown:

[0091] First, the total power P absorbed by each sub-module of the three phases is obtained through the sub-module voltage loop. asm_sum , P bsm_sum , P csm_sum According to the power conservation law, the DC side output power P can be obtained dc , and then calculate the DC side current command value i dcref , and then get the bridge arm current command value i diffx_ref Then, the reference voltage U on the bridge arm reactance is obtained through the bridge arm current loop control. diffx .

[0092] Finally, the upper and lower bridge arm voltage reference values ​​U are obtained through the bridge arm voltage algorithm. xp and U xn The bridge arm voltage expression is:

[0093]

[0094] The voltage balancing methods for the upper and lower bridge arms and the sub-modules within the bridge arms are the same as those on the AC side and will not be described in detail here.

[0095] At this point, the DC-side synchronous charging and voltage-balanced control of the capacitors of the upper and lower bridge arm submodules of the MMC are completed. After that, the normal operation state can be directly entered without the need to design a control strategy and switch.

[0096] For situations where there are requirements for pre-charging speed and charging current, the control strategy proposed in this patent can be combined with the conventional upper and lower bridge arm asynchronous charging control strategy, taking into account the requirements of speed and amplitude, and switching between the two charging control strategies based on the two as measurement indicators to ensure a smooth waveform during the charging process and reduce the electrodynamic impact of core devices, thereby ensuring the stability of the power system.

[0097] The above specific embodiments are only several optional embodiments of the present invention. Based on the technical solutions of the present invention and the relevant inspirations of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A MMC submodule synchronous precharging method based on power balance, characterized in that: Applicable to AC or DC charging scenarios, including: After uncontrolled charging to the limit value using the MMC's own performance, the required power of each phase is calculated by comparing the difference between the actual value and the required value of the MMC submodule voltage, and the reference voltage value of each phase is obtained through the bridge arm voltage algorithm to achieve synchronous charging of the upper and lower bridge arms; Calculate the power difference between the upper and lower bridge arms, and obtain the amplitude representing the fundamental frequency circulating current by comparing the difference between the sum of the average values ​​of the upper and lower bridge arm submodule voltages through the PI controller; The difference between the base frequency circulating current described in the instruction and the actual circulating current is passed through the PR controller, and the physical quantity output by the controller represents the voltage drop on the bridge arm reactor. The voltage difference between the upper and lower bridge arms is compensated to achieve the voltage equalization control of the upper and lower bridge arms. Calculate the power difference of each submodule in the bridge arm, take the average value of the instantaneous value of the capacitor voltage of the submodule in the bridge arm as the command voltage value, multiply the difference between the command voltage value and the actual voltage value by the bridge arm current, and then pass through the proportional controller to obtain the additional reference voltage value of each submodule; Adding the additional reference voltage value to the output voltage reference value of the submodule to obtain an independent reference voltage value for each submodule, thereby achieving capacitor voltage balance of each submodule; After the pre-charging is completed, the normal operation state control is directly entered, and the reference current value is modified from the calculated value during the charging process to the rated value, without the need to design a control strategy and switch it separately.

2. A MMC submodule synchronous precharging method based on power balance according to claim 1, characterized in that: During synchronous pre-charging control on the MMC AC side, the released power of the AC grid is equal to the sum of the charging powers of all three-phase submodules, which can be expressed as: P ac =P asm_sum +P bsm_sum +P bsm_sum ; Where: P ac Represents the power released by the grid, P bsm_sum Represents the total charging power of all submodules in phase B, P csm_sum Represents the total charging power of all sub-modules in phase C.

3. A MMC submodule synchronous precharging method based on power balance according to claim 2, characterized in that, The output power on the AC side of the MMC is reflected in the AC side current, that is; Where: E represents the grid phase voltage amplitude, I represents the grid phase current amplitude.

4. A MMC submodule synchronous precharging method based on power balance according to claim 3, characterized in that: The upper and lower bridge arm voltage reference values ​​U are obtained by the bridge arm voltage algorithm. xp and U xn , expressed as: Where: U xp Indicates the upper bridge arm voltage, U xn Represents the voltage of the lower bridge arm, where x = a, b, c, representing the three phases ABC; U cref Indicates the submodule capacitor voltage command value; U x Indicates the three-phase differential mode voltage reference value.

5. A MMC submodule synchronous precharging method based on power balance according to claim 4, characterized in that: In order to ensure that the voltage difference between the upper and lower bridge arm capacitors is 0, a bridge arm power difference control algorithm is adopted. The bridge arm power difference expression is: ΔP a =-2e a i diffa ; Where: i diffa represents the A phase bridge arm circulation, e a Represents the phase voltage amplitude of phase A.

6. A MMC submodule synchronous precharging method based on power balance according to claim 5, characterized in that: The voltage difference between the upper and lower bridge arm submodules is set to 0, and the feedback value is the sum of the average value of the capacitor voltage of the upper bridge arm submodule (U apc1(avg) +…+U apcN(avg) ) and the sum of the average voltage of the capacitor of the lower bridge arm submodule (U anc1(avg) +…+U ancN(avg) ) is passed through the PI controller, and the output physical quantity represents the amplitude of the fundamental frequency circulating current I diffa1 , the phase of the base frequency circulating current is selected to be in phase with the grid voltage, and then the command base frequency circulating current i diffa1_ref The actual circulation diffa The difference passes through the PR controller, and the physical quantity output represents the voltage drop U on the bridge arm reactor. diffa , the final bridge arm voltage expression is: Where: U diffx Indicates the reference voltage value on the bridge arm reactance.

7. A MMC submodule synchronous precharging method based on power balance according to claim 1, characterized in that: In the synchronous pre-charging control of the MMC DC side, the release power of the DC grid is equal to the sum of the charging powers of all three-phase submodules, which can be expressed as: P dc =P asm_sum +P bsm_sum +P bsm_sum ; Where: P dc Represents the power released on the DC side, P asm_sum Represents the total charging power of all submodules in phase A, P bsm_sum Represents the total charging power of all submodules in phase B, P csm_sum Represents the total charging power of all sub-modules in phase C.

8. A MMC submodule synchronous precharging method based on power balance according to claim 1, characterized in that: The output power on the DC side of the MMC is reflected in the DC side current, that is, the DC side output power P dc It is expressed as; P dc =U dc and dc ; Where: P dc Indicates the DC side output power, U dc Indicates the DC side output voltage, i dc Indicates the DC side output current; Since the three-phase structure of MMC is symmetrical, the DC side current is evenly divided among the three-phase bridge arms, expressed as: Where: i diffa0 、i diffb0 、i diffc0 They represent the DC components in the current of the A-phase bridge arm, the B-phase bridge arm, and the C-phase bridge arm respectively.

9. A MMC submodule synchronous precharging method based on power balance according to claim 8, characterized in that: The upper and lower bridge arm voltage reference values ​​U are obtained by the bridge arm voltage algorithm. xp and U xn The bridge arm voltage expression is:

10. A MMC submodule synchronous precharging method based on power balance according to claim 7 or 9, characterized in that: The independent reference voltage values ​​of the upper and lower bridge arms of the ith submodule are expressed as: Where: U xpsmi_ref and U xnsmi_ref Respectively represent the independent reference voltage values ​​of the i-th submodule of the upper and lower bridge arms; K p Indicates the control coefficient; U xpci and U xnci Respectively represent the independent actual voltage values ​​of the i-th submodule of the upper and lower bridge arms; i xp and i xn Represent the upper and lower bridge arm currents respectively.

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