A self-healing control method for single-phase ground fault in AC / DC hybrid distribution network

Abstract

The invention discloses a single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network, which belongs to the field of AC / DC hybrid distribution network control. When a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point, the method includes: the faulty distribution network side converter adopts an active flexible arc extinguishing control strategy, the neutral point small resistance grounded distribution network side converter adopts a zero-sequence voltage coordinated control strategy, and the remaining distribution network side converters adopt a zero-sequence current suppression strategy; the active flexible arc extinguishing control strategy includes: the converter enters a capacitor current full compensation state, and then switches to a capacitor current under-compensation state; if the fault has been cleared, switch to a fault recovery state; if the fault has not been cleared and is a low-resistance grounding fault, switch to a low-frequency current injection state, and switch to a fault recovery state after the fault is cleared; otherwise, wait for the fault to be cleared before switching to a fault recovery state. It can cope with various types of single-phase grounding faults and improve the power supply reliability of the distribution network.

Description

Technical Field

[0001] The present invention belongs to the field of AC / DC hybrid distribution network control, and more specifically, relates to a single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network. Background Art

[0002] The load-based distribution network is at the end of the power system, directly facing power users, and is responsible for distributing electricity and serving customers. With the continuous maturity of power electronics technology, the cost and loss of power electronic converters are constantly decreasing, and AC / DC hybrid distribution networks connected through converters are becoming the development trend of smart distribution networks. Compared with traditional distribution networks, AC / DC hybrid distribution networks are more intelligent, with a series of advantages such as flexible network topology, high power flow controllability, and strong scalability, making it easier to access new energy.

[0003] At present, the most likely fault in the distribution network is the single-phase grounding fault, which accounts for more than 80% of the total number of distribution network faults. Accidents such as forest fires, burning of power equipment and electric shock caused by single-phase grounding faults in distribution networks occur from time to time. In particular, arc grounding faults are very likely to cause secondary accidents such as ferromagnetic resonance and phase-to-phase short circuits. The fault current of a permanent grounding fault causes a step voltage near the fault point, posing a threat to the safety of people and animals. The traditional solution based on arc suppression coils has the disadvantages of arc overvoltage risk, slow response speed, and poor compensation effect. The traditional solution based on arc suppression cabinets has a large impact on the system when it is put into and withdrawn, and may cause ferromagnetic resonance and other problems. The emergence of active flexible arc suppression technology based on power electronic devices can effectively solve the above problems. However, the traditional active flexible arc suppression technology has problems such as single function, can only be effective in the event of a grounding fault, low utilization efficiency, and high cost.

[0004] In addition, in the case of an AC / DC hybrid distribution network without a connecting transformer, a single-phase ground fault will cause zero-sequence current to flow through each AC distribution network, causing the non-faulty grid to also show fault characteristics, resulting in the relay protection device of the non-faulty grid being falsely triggered and causing the user's power supply to be interrupted. Therefore, how to deal with various ground faults in AC / DC hybrid distribution networks is of great research significance. Summary of the invention

[0005] In view of the defects of the prior art and the need for improvement, the present invention provides a single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network, which aims to cope with various types of single-phase grounding faults and improve the power supply reliability of the distribution network.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present invention, a single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network is provided. When a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point, the method includes: the faulty distribution network side converter adopts an active flexible arc extinguishing control strategy to control its switch tube, the distribution network side converter with a low-resistance grounding at the neutral point adopts a zero-sequence voltage coordinated control strategy to control its switch tube, and the remaining distribution network side converters adopt a zero-sequence current suppression strategy to control their switch tubes; the active flexible arc extinguishing control strategy includes: the converter enters a capacitor current full compensation State, when the full compensation state lasts longer than the preset arc extinguishing time, the converter switches to the capacitor current under-compensation state and determines whether the single-phase grounding fault is cleared; if the single-phase grounding fault has been cleared, the converter switches to the fault recovery state; if the single-phase grounding fault has not been cleared and is a low-resistance grounding fault, the converter switches to the low-frequency current injection state and switches to the fault recovery state after the low-resistance grounding fault is cleared; if the single-phase grounding fault has not been cleared and is a high-resistance grounding fault, the converter switches to the fault recovery state after waiting for the fault to be cleared; the converter switches from the fault recovery state to the normal working state.

[0007] Furthermore, under the active flexible arc extinguishing control strategy, the converter is 0mw1 Control its switch tube; in the state of full compensation of capacitor current, the first QPIR controller is used to control i 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw1 , to i 0g1 Control to i 0ref1 ,i 0ref1 for:

[0008]

[0009] In the capacitor current under-compensation state, the first QPIR controller is used to control i 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw1 , to i 0g1 Control to i 0ref2 ,i 0ref2 for:

[0010]

[0011] In the low-frequency current injection state, the second QPIR controller is used to control i 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw1 , to i 0g1 Control to i 0ref3 ,i 0ref3 for:

[0012]

[0013] Among them, u0mw1 is the modulation wave under the active flexible arc extinguishing control strategy, i 0g1 is the converter zero-sequence current, i 0ref1 is the zero-sequence current reference of the converter under the condition of full compensation of capacitor current, U 0g1 is the zero-sequence voltage amplitude of the AC side grid of the converter implemented by the strategy, r 01 is the single-phase line resistance to ground, θ 0g1 is the phase angle of the zero-sequence voltage of the AC side grid of the converter implemented by the strategy, ω is the fundamental angular frequency, c 01 is the capacitance of the single-phase line to ground, i 0ref2 is the zero-sequence current reference of the converter in the capacitor current under-compensation state, k uc is the capacitor current undercompensation coefficient, i 0ref3 is the zero-sequence current reference under low-frequency current injection, I 0lf is the amplitude of the low-frequency injected current, ω lf is the angular frequency of the low-frequency injected current, and t is the time.

[0014] Furthermore, ω lf satisfy:

[0015]

[0016] Among them, R 0th is the transition resistance threshold using low frequency current injection.

[0017] Furthermore, if R f More than R 0th , the single-phase grounding fault is a high-resistance grounding fault, otherwise, the single-phase grounding fault is a low-resistance grounding fault, R f and R 0th satisfy:

[0018]

[0019] R 0th ≤U 0lfth / I 0lf

[0020] Among them, R f is the transition resistance, R 0th is the transition resistance threshold using low-frequency current injection, is the fault phase voltage phasor, is the zero-sequence voltage phasor, r 01 is the single-phase line resistance to ground, ω is the fundamental angular frequency, c 01 is the capacitance of the single-phase line to ground, is the zero-sequence current phasor, U 0lfth is the low-frequency zero-sequence voltage threshold for low-frequency current injection, I 0lfis the amplitude of the low frequency injected current.

[0021] Furthermore, under the zero-sequence voltage coordinated control strategy, the converter is controlled according to u 0mw3 Control its switch tube, u 0mw3 for:

[0022]

[0023] U 0MMC3 =0.5U png1

[0024] Among them, u 0mw3 is the modulation wave of the zero-sequence voltage cooperative control strategy, U 0MMC3 is the zero-sequence voltage amplitude borne by the converter under zero-sequence voltage cooperative control, θ i0g3 is the zero-sequence current phase angle of the converter under the zero-sequence voltage cooperative control strategy, U png1 is the rated phase voltage amplitude of the distribution network adopting active flexible arc extinguishing control strategy.

[0025] Furthermore, when a single-phase grounding fault occurs in a resonantly grounded distribution network, the method includes: disconnecting the arc suppression coil used for resonant grounding in the faulty distribution network; the faulty distribution network side converter adopts the active flexible arc suppression control strategy to control its switch tube, the distribution network side converter with low resistance grounding at the neutral point adopts the zero-sequence voltage coordinated control strategy to control its switch tube, and the remaining distribution network side converters adopt the zero-sequence current suppression strategy to control their switch tubes.

[0026] Furthermore, when a single-phase grounding fault occurs in a distribution network with a low-resistance grounding at the neutral point, the method includes: the faulty distribution network side converter adopts a zero-sequence voltage feedforward control strategy to control its switch tube, and the remaining distribution network side converters adopt the zero-sequence current suppression strategy to control their switch tubes.

[0027] Furthermore, under the zero-sequence voltage feedforward control strategy, the converter is controlled according to u 0mw3 'Control its switch tube, u 0mw3 'for:

[0028] u 0mw3 ′=k3u 0g3

[0029] k3=(U png3 -0.5U png1 ) / U png3

[0030] Among them, u 0mw3 ′ is the modulation wave output of the converter under the zero-sequence voltage feedforward control strategy, k3 is the second feedforward voltage coefficient, u 0g3is the zero-sequence voltage of the AC grid on the converter side of the zero-sequence voltage feedforward control, U png3 is the rated phase voltage amplitude of the AC grid on the converter side of the zero-sequence voltage feedforward control, U png1 is the rated phase voltage amplitude of the distribution network adopting active flexible arc extinguishing control strategy.

[0031] According to another aspect of the present invention, a single-phase ground fault self-healing control system for an AC / DC hybrid distribution network is provided, comprising: a processor; a memory storing a computer executable program, wherein when the program is executed by the processor, the processor executes the single-phase ground fault self-healing control method for an AC / DC hybrid distribution network as described above.

[0032] According to another aspect of the present invention, a computer-readable storage medium is provided, on which a computer program is stored, and when the program is executed by a processor, the single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network as described above is implemented.

[0033] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0034] (1) A single-phase grounding fault self-healing control method for AC / DC hybrid distribution networks is provided. For the scenario where a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point, the proposed active flexible arc extinguishing strategy can cope with various single-phase grounding faults, such as transient faults, permanent low-resistance faults, and permanent high-resistance faults. It can achieve rapid fault self-healing under most faults, which is beneficial to improving the power supply reliability of the distribution network.

[0035] (2) For the scenario where a single-phase grounding fault occurs in a resonantly grounded distribution network, it is proposed to disconnect the arc suppression coil used for resonant grounding in the faulty distribution network, so that the faulty system is temporarily ungrounded from resonant grounding, thereby implementing a control method for the scenario where a single-phase grounding fault occurs in a distribution network where the neutral point is ungrounded. During the short period of ungrounding, the capacitor current compensation function originally undertaken by the arc suppression coil is fully undertaken by the converter on this side, and the system can operate continuously and safely.

[0036] (3) For the scenario where a single-phase grounding fault occurs in a distribution network with a low-resistance grounding at the neutral point, the proposed zero-sequence current suppression strategy can isolate the faulty grid from the non-faulty grid, thus ensuring the safe operation of the converter and the grid;

[0037] (4) In addition, the proposed zero-sequence voltage coordinated control strategy and zero-sequence voltage feedforward control strategy distribute the zero-sequence voltage required by the AC / DC hybrid distribution network to each converter in proportion, making full use of the redundant voltage capacity of each converter and reducing the design and construction costs of the AC / DC hybrid distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A basic structural diagram of an AC / DC hybrid distribution network provided in an embodiment of the present invention;

[0039] Figure 2 An equivalent circuit diagram of a single-phase grounding fault provided by an embodiment of the present invention when it occurs in a distribution network with an ungrounded neutral point (f1 fault);

[0040] Figure 3 A state switching diagram of each converter when f1 fails provided by an embodiment of the present invention;

[0041] Figure 4 A state switching diagram of each converter when a single-phase grounding fault provided by an embodiment of the present invention occurs in a distribution network with a low resistance grounding at the neutral point (f2 fault);

[0042] Figure 5 A control block diagram of a single-phase ground fault self-healing control method provided by an embodiment of the present invention;

[0043] Figure 6 A variation trend diagram of zero-sequence voltage and zero-sequence current of MMC1 working in an under-compensated state after the f1 fault is cleared provided in an embodiment of the present invention;

[0044] Figure 7 The simulation result of the transient f1 fault provided by the embodiment of the present invention;

[0045] Figure 8 The simulation result of the permanent f1 metallic fault provided by the embodiment of the present invention;

[0046] Fig. 9 The simulation result of the permanent f1 high-resistance fault provided by the embodiment of the present invention;

[0047] Fig.10 The simulation result of the f2 fault provided by the embodiment of the present invention;

[0048] Fig.11 A comparison is made between the method provided in the embodiment of the present invention and the existing method. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0050] In the present invention, the terms "first", "second", etc. (if any) in the present invention and the accompanying drawings are used to distinguish similar objects but not necessarily to describe a specific order or sequence.

[0051] Embodiment 1

[0052] A self-healing control method for single-phase ground fault in AC / DC hybrid distribution network Figure 1-Figure 11 , the single-phase grounding fault self-healing control method for the AC / DC hybrid distribution network in this embodiment is described in detail.

[0053] The grounding method of the neutral point of the distribution network in the AC / DC hybrid distribution network includes low-resistance grounding, in addition, it also includes one or two of the following: no grounding, resonant grounding; or all of them can be low-resistance grounding. Figure 1 Taking the AC / DC hybrid distribution network including three power grids as an example, AC grid 1 and AC grid 2 are ungrounded systems, and AC grid 3 is a low-resistance grounded system. The port converter is generally a modular multilevel converter MMC with a pseudo-bipolar structure. This method is applicable to various types of AC / DC converters. The DC voltage level is generally 20kV, and the AC voltage level is 10kV or 20kV. The AC / DC hybrid distribution network can realize the flexible interconnection of AC distribution networks with different voltage levels and different grounding methods.

[0054] In an AC / DC hybrid distribution network composed of AC distribution networks with different voltage levels and grounding methods interconnected by DC, there are many types and scenarios of single-phase grounding faults. This method proposes that when a single-phase grounding fault occurs, the zero-sequence circuit in the AC / DC hybrid distribution network generates a zero-sequence current, and each converter performs active flexible arc extinguishing, zero-sequence current suppression, zero-sequence voltage coordinated control or zero-sequence voltage feedforward control according to the location where the single-phase grounding fault occurs, so as to be applicable to various single-phase grounding fault scenarios.

[0055] When a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point, the method includes: the faulty distribution network side converter adopts an active flexible arc suppression control strategy to control its switch tube, the distribution network side converter with a low resistance grounding at the neutral point adopts a zero-sequence voltage coordinated control strategy to control its switch tube, and the remaining distribution network side converters adopt a zero-sequence current suppression strategy to control their switch tubes. Ultimately, the self-healing of the single-phase grounding fault in the ungrounded neutral point system is achieved. The equivalent circuit diagram when a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point is as follows: Figure 2 shown.

[0056] It should be noted that the control method when a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point is applicable to the following scenarios: the grounding method of the neutral point of the distribution network in the AC / DC hybrid distribution network is divided into ungrounded and low-resistance grounding; or, it is divided into ungrounded, low-resistance grounding and resonant grounding.

[0057] When a single-phase grounding fault occurs in a distribution network with a low resistance grounding at the neutral point, the method includes: the faulty distribution network side converter uses a zero-sequence voltage feedforward control strategy to control its switch tube, and the remaining distribution network side converters use a zero-sequence current suppression strategy to control their switch tubes. Blocking the propagation path of the fault to the non-fault distribution network system, the method can make full use of the bridge arm voltage capacity of each converter and realize the coordinated suppression of zero-sequence current by each converter.

[0058] It should be noted that the control method when a single-phase grounding fault occurs in a distribution network with a low-resistance grounding at the neutral point is applicable to the following scenarios: the grounding method of the neutral point of the distribution network in the AC / DC hybrid distribution network is low-resistance grounding; or, it is divided into low-resistance grounding and ungrounded; or, it is divided into low-resistance grounding and resonant grounding; or, it is divided into low-resistance grounding, ungrounded and resonant grounding.

[0059] When a single-phase grounding fault occurs in a resonantly grounded distribution network, the method includes: disconnecting the arc suppression coil used for resonant grounding in the faulty distribution network; the faulty distribution network side converter adopts an active flexible arc suppression control strategy to control its switch tube, the distribution network side converter with low-resistance grounding at the neutral point adopts a zero-sequence voltage coordinated control strategy to control its switch tube, and the remaining distribution network side converters adopt a zero-sequence current suppression strategy to control their switch tubes.

[0060] It should be noted that the control method when a single-phase grounding fault occurs in a resonantly grounded distribution network is applicable to the following scenarios: the grounding methods of the neutral points of each AC distribution network in the AC / DC hybrid distribution network are divided into low-resistance grounding and resonant grounding; or, they are divided into ungrounded, low-resistance grounding and resonant grounding.

[0061] In the event of a single-phase grounding fault, the non-fault phase voltage can reach 1.732 times the rated phase voltage. If the zero-sequence voltage component required for zero-sequence current control is provided by one converter, the AC line voltage is 10kV, and the DC voltage is 20kV, the maximum arm voltage can reach 24.14kV (about 18.16kV under normal conditions) in the event of a single-phase grounding fault. This leads to a significant increase in the withstand voltage and capacity of each converter, which in turn leads to a significant increase in the construction cost of the AC / DC hybrid distribution network. Since there are at least two converters in the AC / DC hybrid distribution network with a zero-sequence loop, if multiple converters can coordinately control the zero-sequence current and distribute the zero-sequence voltage between them, the redundant voltage capacity of the converter can be fully utilized and the construction cost can be reduced.

[0062] by Figure 1 Taking the architecture shown in FIG. 1 as an example, when a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point (i.e., AC grid 1), the control mode of each converter is as follows: Figure 3 As shown; when a single-phase grounding fault occurs in a distribution network with a small resistance grounding at the neutral point (i.e., AC grid 3), the control mode of each converter is as follows Figure 4 shown.

[0063] The following specifically describes the active flexible arc extinguishing control strategy, zero-sequence current suppression strategy, zero-sequence voltage coordinated control strategy, and zero-sequence voltage feedforward control strategy in the embodiment of the present invention. Among them, the active flexible arc extinguishing control strategy and the zero-sequence current suppression strategy belong to zero-sequence current control; the zero-sequence voltage coordinated control strategy and the zero-sequence voltage feedforward control strategy belong to zero-sequence voltage control.

[0064] (1) Active flexible arc extinguishing control strategy

[0065] Active flexible arc extinguishing means that the converter outputs zero-sequence current to compensate for the capacitive current to the ground, thereby achieving the purpose of self-extinguishing the arc at the fault point. Figure 3 and Figure 5 ,The active flexible arc extinguishing control strategy is divided into the ,capacitor current full compensation state, capacitor current under-compensation state, low-frequency current injection state and fault recovery state.

[0066] Specifically, under the active flexible arc extinguishing control strategy: the converter first enters the capacitor current full compensation state. When the full compensation state lasts for more than the preset arc extinguishing time, the converter switches to the capacitor current under-compensation state and determines whether the single-phase grounding fault is cleared; if the single-phase grounding fault has been cleared, the converter switches to the fault recovery state; if the single-phase grounding fault has not been cleared and is a low-resistance grounding fault, the converter switches to the low-frequency current injection state and switches to the fault recovery state after the low-resistance grounding fault is cleared; if the single-phase grounding fault has not been cleared and is a high-resistance grounding fault, the converter switches to the fault recovery state after waiting for the fault to be cleared; the converter switches from the fault recovery state to the normal working state.

[0067] Full compensation of capacitor current is used for arc extinguishing under transient faults, and undercompensation of capacitor current is used to estimate transition resistance and realize fault recovery judgment. Low-frequency current injection can realize rapid location and removal of fault lines under low transition impedance. Fault recovery enables the converter to quickly switch from active arc extinguishing strategy to normal control strategy, thereby realizing self-healing of single-phase grounding fault.

[0068] a) Capacitor current full compensation state

[0069] Under the active flexible arc suppression control strategy, the control goal of the converter is to achieve the fault current i by controlling the zero-sequence current. f In the fully compensated state of capacitor current, the phase of current reference depends on the phase of zero-sequence voltage, so a phase-locked loop is used to lock the phase of zero-sequence voltage. 0g1 , the zero-sequence current reference is formula (1):

[0070]

[0071] In order to improve the gain of the fundamental frequency and DC components, the zero-sequence current closed-loop control adopts the first quasi-proportional integral resonant controller (QPIR) to 0g1 Control to i 0ref1 , the transfer function of the controller is:

[0072]

[0073] Furthermore, a partial zero-sequence voltage feedforward k1u is introduced 0g1 To improve the dynamic response performance of the converter, k1 is:

[0074] k1=1-U 0MMC3 / U png1

[0075] Among them, i 0g1 is the converter zero-sequence current, i 0ref1 is the zero-sequence current reference of the converter under the condition of full compensation of capacitor current, U 0g1 is the zero-sequence voltage amplitude of the AC side grid of the converter implemented by the strategy, r 01 is the single-phase line resistance to ground, θ 0g1 is the phase angle of the zero-sequence voltage of the AC side grid of the converter implemented by the strategy, ω is the fundamental angular frequency, c 01 is the capacitance of the single-phase line to ground, G QPIR1 (s) is the transfer function of the first quasi-proportional-integral resonant controller, s is the Laplace operator, k p , k i , k r are proportional coefficient, integral coefficient, resonance coefficient, ω r Influence on the bandwidth of the resonant controller, ω0 is the resonant frequency (such as 100πrad / s), k1 is the first feedforward voltage coefficient, U 0MMC3 is the zero-sequence voltage amplitude borne by the converter under zero-sequence voltage cooperative control, U png1 is the rated phase voltage amplitude of the distribution network adopting active flexible arc extinguishing control strategy.

[0076] Thus, the modulation wave u under the active flexible arc extinguishing control strategy is obtained: 0mw1 , the converter is based on u 0mw1 Control its switch tube.

[0077] b) Capacitor current under-compensation state

[0078] In order to avoid the resonant overvoltage generated by passive arc extinguishing devices and distributed capacitors, most traditional distribution networks use 5%-10% overcompensation of the capacitor current. Active flexible arc extinguishing based on MMC will not cause the resonant overvoltage problem, but if the converter continues to work in the full compensation state, it is difficult to determine whether the f1 fault has been cleared. Therefore, this method proposes active arc extinguishing based on the under-compensation state to perform fault recovery judgment and transition impedance estimation. The zero-sequence current reference is formula (2):

[0079]

[0080] Among them, i 0ref2 is the zero-sequence current reference of the converter in the capacitor current under-compensation state, k uc is the capacitor current undercompensation coefficient, 0.9≤k uc ≤0.95.

[0081] In order to improve the gain of the fundamental frequency and DC component, the zero-sequence current closed-loop control adopts the first QPIR controller to 0g1 Control to i 0ref2 , and further introduces partial zero-sequence voltage feedforward k1u 0g1 To improve the dynamic response performance of the converter. Thus, we get u 0mw1 , the converter is based on u 0mw1 Control its switch tube.

[0082] After entering the capacitor current under-compensation state, determine whether the single-phase grounding fault is cleared. If the single-phase grounding fault is not cleared, it is necessary to further determine whether the fault is a low-resistance grounding fault.

[0083] The judgment method of whether the single-phase grounding fault is cleared is as follows: if the f1 fault is cleared, the zero-sequence voltage source of the faulty distribution network disappears, and the zero-sequence voltage of the distribution network is provided by the converter of the AC / DC hybrid distribution network. Since the converter on the faulty distribution network side only compensates for 90%-95% of the capacitive current under the current zero-sequence voltage at each moment in the under-compensation state, the zero-sequence voltage of the faulty distribution network decreases at each moment, such as Figure 6 As shown in the figure, when the zero-sequence voltage of the faulty distribution network drops to the set threshold value U 0th , it is considered that the f1 fault has been cleared, otherwise, the f1 fault has not been cleared.

[0084] The method for judging whether it is a low-resistance ground fault is as follows: If R f More than R 0th , the single-phase grounding fault is a high-resistance grounding fault, otherwise, the single-phase grounding fault is a low-resistance grounding fault.

[0085] R f and R 0th satisfy:

[0086]

[0087] R 0th ≤U 0lfth / I 0lf

[0088] Among them, R f is the transition resistance, R 0th is the transition resistance threshold using low-frequency current injection, is the fault phase voltage phasor, is the zero-sequence voltage phasor, is the zero-sequence current phasor, U 0lfth is the low-frequency zero-sequence voltage threshold for low-frequency current injection, I 0lf is the amplitude of the low frequency injected current.

[0089] c) Low frequency current injection state

[0090] In the low-frequency current injection state, the frequency selection of the injection signal is particularly critical. If the signal frequency is too high, the injection current will flow into the distributed capacitance. If the frequency is too low, the protection time will be longer. The selected frequency signal needs to make the transition impedance much smaller than the line-to-ground impedance. In engineering, 10 times is often selected as the basis for being much smaller than. The expression is as follows:

[0091]

[0092] The range of injection frequency satisfies:

[0093]

[0094] When low-frequency current is injected, the converter still works in the full compensation state, and the zero-sequence current reference is formula (3):

[0095]

[0096] Among them, i 0ref3 is the zero-sequence current reference under low-frequency current injection, I 0lf is the amplitude of the low-frequency injected current, ω lf is the angular frequency of the low-frequency injection current, t is the time, R 0th is the transition resistance threshold using low frequency current injection.

[0097] In addition, a low-frequency resonant controller is required to achieve zero-error control of the low-frequency current. Therefore, the zero-sequence current closed-loop control uses a second QPIR controller to 0g1 Control to i 0ref3 , the transfer function of the controller is:

[0098]

[0099] Among them, k rlf is also the resonance coefficient. From this, we get u 0mw1 , the converter is based on u 0mw1 Control its switch tube.

[0100] Furthermore, filtering out u 0mw1 The fundamental frequency component is used to extract the low-frequency zero-sequence voltage component U 0lf , when U 0lf Greater than k lf U 0lfth When the single-phase ground fault is cleared, k lf is a constant coefficient greater than 1, and then the converter enters the fault recovery state.

[0101] The control strategy of the fault recovery state is the same as the zero-sequence current suppression strategy in principle. The zero-sequence current suppression strategy will be described later, and the control process of the fault recovery state will not be repeated here.

[0102] (2) Zero-sequence current suppression strategy

[0103] Under the zero-sequence current suppression strategy, the zero-sequence current reference value is 0, and the converter is based on u 0mw2 Control its switch tube, u 0mw2 The acquisition method is: using the second QPIR controller to 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw2 , to i 0g1 The control is zero sequence current reference value 0, where u 0mw2 It is the modulation wave output of the converter under the zero-sequence current suppression strategy.

[0104] (3) Zero-sequence voltage coordinated control strategy

[0105] by Figure 1 Taking the single-phase grounding fault of AC grid 1 in the architecture as an example, MMC1 adopts active flexible arc suppression control strategy, and MMC3 adopts zero-sequence voltage coordinated control strategy. MMC3 cannot directly obtain the zero-sequence voltage of AC grid 1, but the zero-sequence current i 0g1 =-i 0g3 It also flows through MMC3, which contains the zero-sequence voltage phase information of the AC grid 1, so it is necessary to use a phase-locked loop to lock i 0g3 In addition, although the single-phase grounding fault is random and the distribution parameters of different power grids are different, the impedance to ground is resistive and capacitive. Therefore, MMC3 only needs to provide part of the voltage that lags behind i 0g3 The remaining voltage components are provided by the MMC of the faulty power grid. The zero-sequence voltage modulation wave of MMC3 is:

[0106]

[0107] U 0MMC3 The setting should take into account the operating status of the MMC. When MMC1 adopts the active arc extinguishing strategy, the zero-sequence voltage it provides is U png1 -U 0MMC3 When a single-phase ground fault occurs in the AC grid 2, MMC1 adopts a zero-sequence current suppression strategy. If the voltage of the grounding resistance in the AC grid 3 is ignored, MMC1 must also provide a voltage with an amplitude of U 0MMC3 The above analysis is also applicable to MMC2, so the zero-sequence voltage to be provided by MMC2 and MMC1 is:

[0108] U 0MMC1 =U 0MMC2 =max{U png1 -U 0MMC3 ,U 0MMC3}

[0109] In this embodiment, when U 0MMC3 =0.5U png1 When U 0MMC1 and U 0MMC2 Has a minimum value.

[0110] Therefore, under the zero-sequence voltage coordinated control strategy, the converter is controlled according to u 0mw3 Control its switch tube, u 0mw3 for:

[0111]

[0112] U 0MMC3 =0.5U png1

[0113] Among them, u 0mw3 is the modulation wave of the zero-sequence voltage cooperative control strategy, U 0MMC3 is the zero-sequence voltage amplitude borne by the converter under zero-sequence voltage cooperative control, θ i0g3 is the zero-sequence current phase angle of the converter with zero-sequence voltage cooperative control strategy.

[0114] (4) Zero-sequence voltage feedforward control strategy

[0115] by Figure 1 Taking the case of a single-phase grounding fault in the AC grid 3 in the illustrated architecture as an example, MMC1 (MMC2) and MMC3 cooperate to suppress the zero-sequence current, and the current equation of the AC grid 3 is as follows:

[0116]

[0117] Among them, c A3 =c B3 =c C3 =c 03 , rA3 =r B3 =r C3 =r 03 .u 03 is the zero sequence voltage of the AC grid 3. A3 -e C3 is the phase electromotive force of the AC power grid 3. According to the above formula, we can get:

[0118]

[0119] Zero sequence voltage amplitude U of AC grid 3 03 In the worst fault condition, it is approximately equal to its rated phase voltage amplitude. 0MMC1 =U 0MMC2 =0.5U png1 , the maximum zero-sequence voltage amplitude that MMC3 must provide is:

[0120] U 0MMC3 ′=U png3 -0.5U png1

[0121] In addition, the zero-sequence voltage feedforward control strategy is applied to MMC3. The zero-sequence voltage provided during the f2 fault is proportionally distributed between MMC3 and other MMCs. The proportional factor k3 can be calculated as follows:

[0122] k3=U 0MMC3 ′ / U png3 =(U png3 -0.5U png1 ) / U png3

[0123] Therefore, under the zero-sequence voltage feedforward control strategy, the converter is controlled according to u 0mw3 'Control its switch tube, u 0mw3 'for:

[0124] u 0mw3 ′=k3u 0g3

[0125] k3=(U png3 -0.5U png1 ) / U png3

[0126] Among them, u 0mw3 ′ is the modulation wave output of the converter under the zero-sequence voltage feedforward control strategy, k3 is the second feedforward voltage coefficient, u 0g3 is the zero-sequence voltage of the AC grid on the converter side of the zero-sequence voltage feedforward control, U png3 is the rated phase voltage amplitude of the AC grid on the converter side of the zero-sequence voltage feedforward control.

[0127] In order to verify the effectiveness of this method, a Figure 1 The AC / DC hybrid distribution network model is shown, the parameters of the AC / DC hybrid distribution network are shown in Table 1, and the parameter settings of the MMC are shown in Table 2.

[0128] Table 1

[0129] Parameter name Numeric Parameter name Numeric AC grid 1 voltage 10kV DC voltage 20kV AC grid 2 voltage 10kV <![CDATA[Ground capacitance c 01 > 20μF AC grid 3 voltage 20kV <![CDATA[Transition resistance R f > 1Ω or 100Ω Grounding resistance of AC grid 3 5Ω <![CDATA[Ground resistance r 01 > 50kΩ

[0130] Table 2

[0131] Parameter name Numerical value of MMC1 MMC2 Values MMC3 Values Power capacity 10MVA 10MVA 10MVA Rated AC voltage 10kV 10kV 20kV Rated DC voltage 20kV 20kV 20kV Submodule capacitor voltage 2500V 2500V 4000V Controller sampling frequency 10kHz 10kHz 10kHz Bridge arm inductance 10mH 10mH 10mH Submodule capacitance value 4.7mF 4.7mF 4.7mF Number of submodules 60 60 60

[0132] MMC1 and MMC2 operate in PQ mode, and MMC3 operates in VQ mode, thereby realizing power interaction between the AC / DC hybrid distribution network and the three AC grids. Before and after the single-phase grounding fault, MMC1 and MMC2 feed 2MW of active power into AC grid 1 and AC grid 2 respectively. Furthermore, the A-phase grounding fault is simulated in AC grid 1 and AC grid 3 respectively. Set U 0lfth 500V, I 0lf is 10A, then the maximum transition resistance under low-frequency current injection is R 0th =50Ω. Set the frequency of the low-frequency injection current to 5Hz.

[0133] Figure 7-Figure 10 The simulation results are shown. Figure 7 , Figure 8 and Fig. 9 In the figure, (a) represents the three-phase voltage to ground of AC grid 1; (b) represents the zero-sequence voltage of AC grid 1; (c) represents the three-phase output current of MMC1; (d) represents the ground current at the fault point; (e) represents the zero-sequence current i of MMC1. 0g1 ; (f) represents the zero-sequence current i of MMC2 and MMC3 0g2 and i 0g3 . Fig.10 In the figure, (a) represents the three-phase voltage to ground of AC grid 1; (b) represents the zero-sequence voltage of AC grid 1; (c) represents the three-phase output current of MMC1; (d) represents the zero-sequence current i of MMC1. 0g1 ; (e) represents the zero sequence current i of MMC3 0g3 ; (f) represents the zero-sequence current i of MMC2 0g2 .

[0134] Figure 7The simulation results when a transient f1 fault occurs in the AC power grid 1 are shown, and the fault duration is 0.4-0.6s. After the fault occurs, MMC1 enters the capacitor current full compensation state, the output compensation current amplitude is 52A, and the fault point current amplitude is about 0A. After the fault is cleared, MMC1 is still in the capacitor current full compensation state for a short time. After reaching the set time (300ms) of the capacitor current full compensation state, MMC1 enters the capacitor current undercompensation state at 0.7s, and the zero-sequence voltage and current gradually decrease. At 0.81s, MMC1 enters the fault recovery state, and finally realizes the instantaneous f1 fault self-healing.

[0135] Figure 8 The simulation results are shown when a permanent f1 fault (metallic fault) occurs in the AC power grid 1, and the fault duration is 0.4-1.2s. A metallic ground fault (R f =1Ω), MMC1 immediately enters the state of full compensation of capacitor current, realizing full compensation of capacitor current at the fault point. At this time, the compensation current amplitude output by MMC1 is 52A, and the current amplitude at the fault point is less than 1A. Then MMC1 enters the state of under-compensation of capacitor current at 0.7s, at which time the compensation current amplitude is 46A, and the current amplitude at the fault point is 14.5A. When the transition resistance R f <R 0th When the f1 fault is cleared at 1.2s, the low-frequency zero-sequence voltage injected by MMC1 is greater than the threshold value k. lf U 0lfth , it enters the fault recovery state and finally achieves permanent f1 fault self-healing.

[0136] Fig. 9 The simulation results are shown when a permanent f1 fault (high resistance fault) occurs in the AC power grid 1, and the fault duration is 0.4-1s. A high resistance ground fault (R f =100Ω), MMC1 immediately enters the capacitor current full compensation state, and enters the capacitor current under compensation state after 300ms. f >R 0th , so MMC1 will continue to remain in the capacitor current undercompensation state. When the fault is manually cleared, MMC1 automatically enters the fault recovery state at about 1.11s and finally returns to the normal working state.

[0137] In the above types of f1 faults, MMC2 is always in the zero-sequence current suppression state, such as Figure 7-Figure 9As shown in (f), the normal operation of the AC power grid 2 is not affected. MMC3 is always in the zero-sequence voltage coordinated control state.

[0138] Fig.10 The simulation results (R f =1Ω), the fault duration is 0.4-0.8s. When the AC grid 3 has a ground fault in phase A at 0.4s, MMC1 and MMC2 immediately enter the zero-sequence current suppression state, and MMC3 enters the zero-sequence voltage feedforward control state to block the zero-sequence current. Fig.10 It can be found that the zero-sequence voltage of AC grid 1 approaches 0. After the relay device removes the f2 fault (0.8s), all MMCs return to normal working state.

[0139] Furthermore, the zero-sequence voltage borne by MMC1-MMC3 in each single-phase grounding fault is analyzed, such as Fig.11 As shown. The zero-sequence current control strategy of the AC / DC hybrid distribution network proposed by the existing method only uses a single MMC on the fault side, so each MMC needs to bear all the zero-sequence voltage of the faulty grid, that is, the rated phase voltage. Since this method uses multiple MMCs to share the zero-sequence voltage (zero-sequence voltage cooperative control and zero-degree voltage feedforward control strategy), it can significantly reduce the voltage capacity required by each converter under a single-phase grounding fault. Under f1 fault, MMC1, MMC2 and MMC3 only need to provide half of the zero-sequence voltage in AC grid 1. Under f2 fault, MMC1 and MMC2 still provide the same zero-sequence voltage, that is, one-fourth of the zero-sequence voltage in AC grid 2. At the same time, MMC3 only needs to provide three-quarters of the zero-sequence voltage in AC grid 2. Assuming that the construction cost of each MMC unit voltage capacity is the same, this method reduces the construction cost by 37.5% compared with the existing method. With the increase in the number of MMCs in the future AC / DC hybrid distribution network, the control strategy proposed by this method will greatly reduce the construction cost of MMC.

[0140] Embodiment 2

[0141] An electronic device includes: a processor; a memory storing a computer executable program, wherein when the program is executed by the processor, the processor executes the above-mentioned single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network. The related technical solution is the same as that of the first embodiment, and will not be described in detail here.

[0142] Embodiment 3

[0143] A computer-readable storage medium stores a computer program, and when the program is executed by a processor, the single-phase ground fault self-healing control method for an AC / DC hybrid distribution network is implemented. The related technical solution is the same as that of the first embodiment, and will not be described in detail here.

[0144] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A single-phase ground fault self-healing control method for an AC / DC hybrid distribution network, characterized in that: When a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point, the methods include: The faulty distribution network side converter adopts active flexible arc suppression control strategy to control its switch tube, the distribution network side converter with low resistance grounding at the neutral point adopts zero-sequence voltage coordinated control strategy to control its switch tube, and the remaining distribution network side converters adopt zero-sequence current suppression strategy to control their switch tubes; The active flexible arc extinguishing control strategy includes: The converter enters the capacitor current full compensation state. When the full compensation state lasts longer than the preset arc extinguishing time, the converter switches to the capacitor current under-compensation state and determines whether the single-phase grounding fault is cleared. If the single-phase ground fault has been cleared, the converter switches to the fault recovery state; If the single-phase grounding fault is not cleared and is a low-resistance grounding fault, the converter switches to the low-frequency current injection state, and switches to the fault recovery state after the low-resistance grounding fault is cleared; if the single-phase grounding fault is not cleared and is a high-resistance grounding fault, the converter switches to the fault recovery state after the fault is cleared; The converter switches from a fault recovery state to a normal working state; Under the active flexible arc extinguishing control strategy, the converter is based on u 0mw1 Control its switch tube; In the state of full compensation of capacitor current, the converter uses the first QPIR controller to 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw1 , to i 0g1 Control to i 0ref1 ,i 0ref1 for: In the capacitor current under-compensation state, the converter uses the first QPIR controller to 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw1 , to i 0g1 Control to i 0ref2 ,i 0ref2 for: In the low-frequency current injection state, the converter uses the second QPIR controller to 0g1 Perform zero-sequence current closed-loop control to obtain u 0mw1 , to i 0g1 Control to i 0ref3 ,i 0ref3 for: Among them, u 0mw1 is the modulation wave under the active flexible arc extinguishing control strategy, i 0g1 is the converter zero-sequence current, i 0ref1 is the zero-sequence current reference of the converter under the condition of full compensation of capacitor current, U 0g1 is the zero-sequence voltage amplitude of the AC side grid of the converter implemented by the strategy, r 01 is the single-phase line resistance to ground, θ 0g1 is the phase angle of the zero-sequence voltage of the AC side grid of the converter implemented by the strategy, ω is the fundamental angular frequency, c 01 is the capacitance of the single-phase line to ground, i 0ref2 is the zero-sequence current reference of the converter in the capacitor current under-compensation state, k uc is the capacitor current undercompensation coefficient, i 0ref3 is the zero-sequence current reference under low-frequency current injection, I 0lf is the amplitude of the low-frequency injected current, ω lf is the angular frequency of the low-frequency injected current, and t is the time.

2. The method according to claim 1, characterized in that ω lf satisfy: Among them, R 0th is the transition resistance threshold using low frequency current injection.

3. The method according to claim 1, characterized in that If R f More than R 0th , the single-phase grounding fault is a high-resistance grounding fault, otherwise, the single-phase grounding fault is a low-resistance grounding fault, R f and R 0th satisfy: R 0th ≤U 0lfth / I 0lf Among them, R f is the transition resistance, R 0th is the transition resistance threshold using low-frequency current injection, is the fault phase voltage phasor, is the zero-sequence voltage phasor, r 01 is the single-phase line resistance to ground, ω is the fundamental angular frequency, c 01 is the capacitance of the single-phase line to ground, is the zero-sequence current phasor, U 0lfth is the low-frequency zero-sequence voltage threshold for low-frequency current injection, I 0lf is the amplitude of the low frequency injected current.

4. The method according to claim 1, characterized in that Under the zero-sequence voltage coordinated control strategy, the converter is controlled according to u 0mw3 Control its switch tube, u 0mw3 for: The 0MMC3 =0.5U png1 Among them, u 0mw3 is the modulation wave of the zero-sequence voltage cooperative control strategy, U 0MMC3 is the zero-sequence voltage amplitude borne by the converter under zero-sequence voltage cooperative control, θ i0g3 is the zero-sequence current phase angle of the converter under the zero-sequence voltage cooperative control strategy, U png1 is the rated phase voltage amplitude of the distribution network adopting active flexible arc extinguishing control strategy.

5. The method according to any one of claims 1 to 4, characterized in that: When a single-phase ground fault occurs in a resonantly grounded distribution network, the methods include: Disconnect the arc suppression coil used for resonant grounding in the faulty distribution network; The faulty distribution network side converter adopts the active flexible arc extinguishing control strategy to control its switch tube, the distribution network side converter with low resistance grounding at the neutral point adopts the zero-sequence voltage coordinated control strategy to control its switch tube, and the remaining distribution network side converters adopt the zero-sequence current suppression strategy to control their switch tubes.

6. The method according to any one of claims 1 to 4, characterized in that: When a single-phase grounding fault occurs in a distribution network with a low-resistance grounding at the neutral point, the methods include: The faulty distribution network side converter adopts the zero-sequence voltage feedforward control strategy to control its switch tube, and the remaining distribution network side converters adopt the zero-sequence current suppression strategy to control their switch tubes.

7. The method according to claim 6, characterized in that Under the zero-sequence voltage feedforward control strategy, the converter is controlled according to u 0mw3 'Control its switch tube, u 0mw3 'for: you 0mw3 ′=k3u 0g3 k3=(Upng3-0.5Upng1) / Upng3 Among them, u 0mw3 ′ is the modulation wave output of the converter under the zero-sequence voltage feedforward control strategy, k3 is the second feedforward voltage coefficient, u 0g3 is the zero-sequence voltage of the AC grid on the converter side of the zero-sequence voltage feedforward control, U png3 is the rated phase voltage amplitude of the AC grid on the converter side of the zero-sequence voltage feedforward control, U png1 is the rated phase voltage amplitude of the distribution network adopting active flexible arc extinguishing control strategy.

8. A single-phase ground fault self-healing control system for an AC / DC hybrid distribution network, characterized in that: include: processor; A memory storing a computer executable program, wherein when the program is executed by the processor, the processor executes the single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network as described in any one of claims 1 to 7.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, a single-phase grounding fault self-healing control method for an AC / DC hybrid distribution network is implemented according to any one of claims 1 to 7.

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