Fault recovery method for ultra-high voltage hybrid line based on controllability of direct current

Abstract

This invention discloses a fault recovery method for ultra-high voltage hybrid transmission lines based on DC current controllability. First, after line deionization, current is injected as the controlled variable, fundamentally avoiding overcurrent problems that may be caused by voltage injection. Second, the differences in the control response of the hybrid MMC to DC current are analyzed based on the equivalent circuit of the system under different fault characteristics, thus forming a fault recovery strategy based on DC current controllability. Finally, simulations verify the effectiveness of the proposed strategy. The results show that the proposed strategy can accurately and reliably identify the fault characteristics under various fault conditions, and does not require communication, thus improving the power supply reliability of the system.

Description

Technical Field

[0001] This invention relates to the field of power line technology, specifically to a fault recovery method for ultra-high voltage hybrid lines based on the controllability of direct current. Background Technology

[0002] High-voltage direct current (HVDC) transmission is a crucial means of solving the problem of large-capacity, long-distance power transmission and plays a vital role in energy optimization. Among them, hybrid HVDC systems, which combine the advantages of conventional line commutated converter-high voltage direct current (LCC-HVDC) and flexible multilevel converter-high voltage direct current (MMC-HVDC), have broad development prospects in the field of HVDC transmission. From the current technological perspective, considering factors such as the manufacturing cost of DC circuit breakers and valve group insulation, a single-pole double-valve group connection method is recommended for ultra-high voltage (UHVDC) hybrid HVDC systems. Furthermore, a submodule with DC fault self-clearing capability should be used on the inverter side to achieve DC fault ride-through and online valve group activation / deactivation functions. Currently, some UHV hybrid DC transmission projects with DC fault ride-through capability have been completed (such as the "Kunliulong" ±800kV UHV hybrid DC project) or are under construction (such as the Kubuqi North-Cangzhou ±800kV UHV hybrid DC project). These projects all adopt full / half-bridge hybrid MMC with negative voltage output capability on the inverter side.

[0003] In long-distance, high-capacity power transmission, rapid recovery from DC line faults is crucial for ensuring equipment safety, improving power supply reliability, and maintaining the stability of the entire system. In practical engineering, hybrid DC transmission systems utilize the fault recovery strategies of traditional high-voltage DC transmission systems and flexible DC transmission systems. For the LCC side, fault clearing and recovery are achieved through the DC line fault recovery sequence (DFRS). For the MMC side, which has self-clearing capabilities, the fault recovery method involves locking the converter to clear the fault current and undergoing a deionization process, then unlocking the converter and checking whether the DC voltage can be established normally to determine if the fault has disappeared. However, these recovery strategies lack prior fault identification. Restarting a permanent fault can cause more severe secondary impacts on the system, which is detrimental to the safe and stable operation of the system. Furthermore, to accurately distinguish between permanent and transient faults with large transition resistance, a sufficiently large voltage detection threshold must be set, which means a longer detection (recovery) time is required. For example, the restart detection time in the Yongfu LCC-HVDC project is set to 75ms.

[0004] To address this issue, numerous scholars have proposed methods for fault nature identification in DC transmission lines. For UHV hybrid DC transmission systems employing hybrid MMCs on the inverter side, some experts have pointed out that a sinusoidal current signal can be injected into the line using an LCC. Based on Kirchhoff's current law, the difference in current similarity between the two ends of the line under different fault natures can be obtained, and a dynamic time warping algorithm can be introduced to measure this difference. This method has strong resistance to transition resistance and anti-interference capabilities, but it requires the transmission of a large amount of current data, placing a heavy burden on the communication system. To avoid relying on communication, some experts have proposed using a hybrid MMC to output a low-frequency characteristic signal to the line. By establishing a time-domain distributed parameter model of the line, the calculated voltage at the other end can be obtained and compared with the voltage at the local end to determine the fault nature. However, this method is not good at distinguishing between transient / permanent faults located at the converter outlet, and the calculation accuracy is highly dependent on the accurate acquisition of line parameters. Other scholars have proposed establishing a fault loop equation that includes fault distance and fault impedance to solve for the fault distance, and determining the fault nature based on the polarity of the fault distance. Theoretically, for port faults located at the beginning of the line, the fault distance is zero, and the reliability of this method needs further verification.

[0005] To address the aforementioned issues, and with the goal of improving the system restart success rate, a fault recovery strategy based on current injection is proposed from the perspective of control and protection coordination. Summary of the Invention

[0006] The purpose of this invention is to provide a fault recovery method for ultra-high voltage hybrid lines based on the controllability of DC current, thereby solving the requirements of the above-mentioned background technology.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] The fault recovery method for ultra-high voltage hybrid lines based on the controllability of DC current includes the following steps:

[0009] Step 1: After receiving the line protection action signal, the LCC station performs a phase shift operation. After the phase shift process is completed, it collects the voltage u at the protection installation point at the beginning of the line. dcR Data substitution u dcR (t)>u Δ (10) Check if the voltage has been rebuilt, u Δ To set the threshold, the hybrid MMC station switches to DC current suppression mode after receiving a line protection action signal. At the same time, the counter n=1 is set, indicating that the first injection is about to begin.

[0010] Step 2: After 300ms of deionization, the hybrid MMC switches to current injection mode and attempts to inject a step current signal with an amplitude of 0.5pu into the line.

[0011] Step 3: Acquire the m of the PI output during the time period from 2τ to 2τ+T after injection. dc Substitute the data into (9) to determine the nature of the fault. If m dc Failed to satisfy the formula within 3 consecutive sampling points In the formula, the DC modulation ratio m dc The output of the PI loop in the control system is obtained, where T is the data window length of the identification criterion, taken as T = 5ms; τ = l / v, where l is the line length, v is the traveling wave propagation speed, and m is the speed of light; Δ To determine the threshold, take m. Δ =0.85; and n <n max n max To maximize the number of injections, let n = n + 1. The hybrid MMC switches to current suppression mode and jumps to step two. If m... dc Equation (9) could not be satisfied within 3 consecutive sampling points, and n = n max Then, the hybrid MMC is locked, if m dc When three consecutive sampling points satisfy equation (9), the hybrid MMC immediately switches back to the original DC voltage control;

[0012] Step 4: During steps 2 and 3, determine u in real time. dcR If equation (10) is satisfied, the LCC immediately switches back to the original DC current control to restore the system current. If the protection action occurs after (n) max If equation (10) cannot be satisfied within the +1)×300ms time period, then the LCC is blocked.

[0013] As a further technical solution of the present invention: Based on an ultra-high voltage hybrid DC transmission system, each pole of the ultra-high voltage hybrid DC transmission system adopts a dual-valve group structure with symmetrical parameters between the two poles. On the rectifier side, LCC converter (i.e., dual 12-pulse series connection) is used, and on the inverter side, a hybrid MMC converter (full / half bridge submodule) is used. When the system is operating normally, the LCC station uses constant DC current control, and the MMC station uses constant DC voltage / reactive power control. The voltage and current at the protection installation point at the beginning of the line are denoted as u. dcR i dcR The voltage and current at the protection installation point at the end of the line are denoted as u. dcI i dcI .

[0014] As a further technical solution of the present invention: For LCC stations, when a fault occurs in the DC line, the DC line protection should act quickly. For LCC stations, after the pole control system receives the line protection action signal, the thyristor firing angle is urgently shifted to 120°, causing the LCC to switch from rectification mode to inverter operation, so that the fault energy in the DC system is fed back to the AC grid, reducing the current i flowing into the fault point from the LCC. dcR , when i dcR When the trigger angle is reduced to near zero, the trigger angle is further phase-shifted to over 150°.

[0015] As a further technical solution of the present invention: For LCC station and MMC station, active control of fault current is achieved by switching control mode. Thanks to the negative voltage output capability of the full bridge submodule, the DC voltage of the hybrid MMC has a certain adjustment margin. On the one hand, during DC fault ride-through, the DC voltage is adjusted using this adjustment margin to suppress the fault current output to the short circuit point. On the other hand, during DC fault recovery, a small-amplitude characteristic signal is injected into the DC line using this adjustment margin to help determine whether the fault point has disappeared.

[0016] As a further technical solution of the present invention, it also includes electrical quantity response characteristic analysis under different fault types, specifically including electrical quantity analysis of permanent faults and transient fault current characteristic analysis.

[0017] As a further technical solution of the present invention: the electrical quantity analysis of the permanent fault includes current characteristic analysis and voltage characteristic analysis.

[0018] As a further technical solution of the present invention: the transient fault current characteristic analysis includes current characteristic analysis and voltage characteristic analysis.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] In hybrid DC transmission systems using overhead lines as power transmission channels, DC line faults are inevitable, necessitating safe, fast, and reliable fault recovery strategies. This paper addresses the lack of fault nature discrimination in existing recovery strategies. By fully utilizing the control characteristics of hybrid MMC (Multi-Mode Controlled Transmission) systems, aiming to improve system restart success rate and reduce secondary impacts, a step current is injected into the line after deionization to reflect the system's dynamic characteristics under different fault types. Therefore, a DC line fault recovery strategy based on the controllability of DC current is proposed. The main contributions are as follows:

[0021] 1) The use of current as the controlled variable effectively prevents overcurrent surges caused by characteristic signal injection under converter outlet faults.

[0022] 2) The response characteristics of electrical quantities after signal injection were analyzed based on the equivalent circuit of the system under different fault types: For transient faults, the inverter side current is uncontrollable and the rectifier side voltage is the system voltage; for permanent faults, the inverter side current is controllable and the rectifier side voltage is proportional to the transition resistance.

[0023] 3) Using the control signal output by the PI as a characteristic quantity, a fault characteristic identification criterion based on the DC voltage modulation ratio was constructed on the inverter side.

[0024] 4) Extensive simulation results demonstrate that the proposed fault recovery strategy can adapt to various fault conditions occurring throughout the entire line, withstand transition resistance up to 500Ω, tolerate noise with a signal-to-noise ratio of 30dB, and is insensitive to changes in system parameters, exhibiting good versatility. Furthermore, the fault recovery strategy does not rely on a communication system and has low requirements for the sampling rate of the protection device, making it easy to implement in engineering applications. Attached Figure Description

[0025] Figure 1 This is a topology diagram of an ultra-high voltage hybrid direct current transmission system.

[0026] Figure 2 This is a diagram of the hybrid MMC fault ride-through and current injection control strategy.

[0027] Figure 3 It is the equivalent circuit diagram of the system under permanent fault conditions.

[0028] Figure 4 It is a block diagram of the control system.

[0029] Figure 5 Under permanent fault i dcI Waveform diagram.

[0030] Figure 6 Under permanent fault conditions u dcR Waveform diagram.

[0031] Figure 7It is the equivalent circuit diagram of the system under transient faults.

[0032] Figure 8 It is the i dcI waveform diagram under transient faults.

[0033] Figure 9 It is the equivalent circuit diagram when t0 < t < t0 + 2τ under transient faults.

[0034] Figure 10 It is the u dcR waveform diagram under transient faults.

[0035] Figure 11 It is the system recovery flowchart.

[0036] Figure 12 It is the m dc waveform diagram when transient faults occur at three locations.

[0037] Figure 13 It is the u dcR waveform diagram when transient faults occur at three locations.

[0038] Figure 14 It is the i dcR waveform diagram when transient faults occur at three locations.

[0039] Figure 15 It is the permanent fault m dc waveform diagram.

[0040] Figure 16 It is the permanent fault u dcR waveform diagram.

[0041] Figure 17 It is the frequency-variable parameters and tower model diagram of the DC overhead line. Specific implementation manners

[0042] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0043] As Figure 1-17As shown, a fault recovery method for UHV hybrid lines based on the controllability of DC current is proposed. First, the control responses of the LCC and hybrid MMC substations after DC line protection operation are analyzed, and the injection method of the current signal on the hybrid MMC side is given. Second, based on the system circuit model and control system model, the controllability of the DC current after injection control switching is studied, and the differences in voltage and current response characteristics under different fault types are analyzed in depth. Based on these differences, a fault type identification criterion is constructed, and a system recovery strategy is further formulated. Finally, the effectiveness of the proposed fault recovery strategy is verified by simulation in PSCAD.

[0044] Figure 1 The diagram shows the topology of an ultra-high voltage hybrid direct current (UHVDC) transmission system. Each pole of this system employs a dual-valve group structure with symmetrical parameters. The rectifier side uses LCC converters (dual 12-pulse series connection), and the inverter side uses a hybrid MMC converter (full / half bridge submodule). During normal system operation, the LCC station uses constant DC current control, and the MMC station uses constant DC voltage / reactive power control. In the diagram, f1, f2, and f3 represent fault locations at the beginning, middle, and end of the line, respectively (all faults within the DC line area, requiring fault nature determination). Let u be the denoteed value. dcR i dcR These represent the voltage and current at the protection installation point at the beginning of the line, respectively; u dcI i dcI These are the voltage and current at the protection installation point at the end of the line, respectively. The positive direction for measuring the current is specified as pointing from the busbar towards the line.

[0045] The converter's control response after the DC line protection trips:

[0046] LCC Station Phase Shift Control: When a fault occurs on the DC line, the DC line protection should operate rapidly. For LCC stations, after receiving the line protection operation signal, the pole control system urgently shifts the thyristor firing angle to 120°. This switches the LCC from rectification mode to inverter operation, allowing the fault energy in the DC system to be fed back to the AC grid, reducing the current i flowing into the fault point from the LCC. dcR . when i dcR When the trigger angle is reduced to near zero, the trigger angle is further phase-shifted to over 150°.

[0047] Hybrid MMC station DC current control:

[0048] For hybrid MMC substations, active control of fault current can be achieved by switching control modes. Thanks to the negative voltage output capability of the full-bridge submodule, the hybrid MMC DC voltage has a certain adjustment margin. On the one hand, during DC fault ride-through, adjusting the DC voltage using this adjustment margin can suppress the fault current output to the short-circuit point. On the other hand, during DC fault recovery, this adjustment margin can be used to inject a small-amplitude characteristic signal into the DC line to help determine whether the fault point has disappeared, avoiding blind restarting during the fault recovery process.

[0049] Some literature uses a step voltage signal with an amplitude of 0.1 pu as the injection signal. According to circuit laws, when the voltage is fixed, the fault current is inversely proportional to the impedance. Furthermore, since the hybrid MMC has fault current control capability, the inductance of the current-limiting reactor configured at its DC outlet is generally small to reduce investment costs and improve system dynamic response. Considering extreme cases, when a metallic bipolar short-circuit fault occurs at the hybrid MMC outlet, it can easily cause overcurrent in the converter valve. Therefore, this paper selects a current signal as the injection signal to assist the protection system in identifying the fault nature.

[0050] Hybrid MMC control strategies including current signal injection control, such as Figure 2 As shown.

[0051] Figure 2 All quantities are per-unit values, where U dcref This is the DC voltage reference value. Under normal system operation, U... dcref =1.0pu;i dcIpu and i dcref These are the measured DC current and the DC current command value of the inverter station, respectively; u diffj and u cirj These are the reference values ​​for the differential mode voltage and the circulating current voltage, respectively; u p(n)jref This is the reference value for the upper (lower) bridge arm voltage of phase j. DC current control is implemented using a PI controller, and the PI output is m. dc The DC current modulation ratio is expressed as shown in equation (1).

[18] :

[0052]

[0053] In the formula, u MMC It features a wide adjustable range for the combined upper and lower bridge arm voltages of the MMC; U N This is the rated DC voltage.

[0054] Depend on Figure 2 As can be seen, the switching and coordination of S1 and S2 allows the DC control loop to offer three selectable control modes. These three control modes are explained below:

[0055] Mode I: DC voltage control mode. In this mode, both switches S1 and S2 are set to the 0 position.

[0056] Mode II: DC Current Suppression Mode. After a DC line fault, the line protection will activate on a millisecond timescale, switching control switch S1 from 0 to 1 (S2 remains at 0), and the hybrid MMC enters DC current control mode. In this mode, the hybrid MMC DC voltage will adapt to changes in DC current until the DC current reaches the preset value 0p.u.

[0057] Mode III: DC Current Injection Mode. After the fault current drops to 0, and after a certain deionization time, control switch S2 switches from 0 to 1 (S2 remains in position 1). The hybrid MMC attempts to inject a step signal with an amplitude transiently changing from 0 p.u. to 0.5 p.u. into the DC line. Generally, the maximum overcurrent capacity within the MMC is twice the rated current; therefore, using a current signal with an amplitude of 0.5 p.u. can effectively reduce the overcurrent risk of the MMC.

[0058] Hybrid MMC enables decoupled AC / DC control, and switching between the three control modes in the DC control loop does not affect the AC control loop. Furthermore, the protection action strategy does not lock out the hybrid MMC; therefore, during DC line fault identification, the hybrid MMC can continuously provide reactive power support to the receiving-end AC grid.

[0059] Analysis of electrical quantity response characteristics under different fault types: The response characteristics of measured current and voltage should be determined by the simultaneous application of control equations and circuit equations. Clearly, the circuit structure differs between permanent and transient faults (essentially, whether or not a faulty branch is present). Figure 2 When injection control is applied, the electrical quantity response characteristics will inevitably differ.

[0060] Electrical quantity analysis of permanent faults:

[0061] Current characteristic analysis:

[0062] The above analysis shows that LCC, before switching from hybrid MMC to injection mode, I dcR =0, therefore, in the equivalent circuit diagram, the rectifier side can be treated as an open circuit, thus obtaining as follows: Figure 3 The equivalent circuit of the system under a permanent fault is shown in the figure. In the figure, f represents the fault point, and R... f This represents the transition resistance at the fault point; terminals R and I represent the beginning and end of the line, respectively; α is the proportion of the distance from R to the fault point to the total length of the line; R l and L l The equivalent resistance and inductance per unit length of the line; L p and L x These are the smoothing reactor inductors for LCC stations and the current-limiting reactor inductors for hybrid MMC stations; LMMC =2L arm / 3, R MMC =2R arm / 3,L arm and R arm These are the hybrid MMC bridge arm inductors and their equivalent resistances, respectively.

[0063] According to equation (1) and Figure 3 The circuit shown has a DC current (per unit value) under a permanent fault, as shown in equation (1):

[0064]

[0065] In the formula, I N R is the rated current; eq =R MMC +(1-α)R x L eq =L MMC +(1-α)L x ;R eqpu =R eq / Z N L eqpu =L eq / Z N Z N =U N / I N This is the impedance reference value.

[0066] Combination Figure 2 The block diagram of the DC current control loop can be drawn using equation (2) as follows: Figure 4 As shown.

[0067] Depend on Figure 4 It can be seen that by properly setting the parameters of the PI controller, closed-loop control of the DC current can be achieved, making the actual current i dcIpu Effective tracking i dcref The control equation for the direct current is equation (3):

[0068]

[0069] In the formula, k p and k i These are the proportional and integral coefficients of the PI controller, respectively. In this paper, k... p =5,k i =10 3 .

[0070] Eliminating M by combining equations (2) and (3) dc The closed-loop transfer function for applying current injection control under permanent fault conditions is shown in equation (4):

[0071]

[0072] Considering that the step signal has a stable DC component and energy distribution under steady-state conditions, the step current signal is selected as the characteristic signal for injection in Section 2.2. Before injection control switching (denoted as t...). 0- ), there is i dcIpu (t 0- ) = i dcref (t 0- =0p.u., after injection control switching (denoted as t) 0+ ), there is i dcref (t 0+ ) = 0.5pu. Therefore, in equation (4), i dcref (s)=0.5 / s. In summary, the DC current response under permanent fault with current injection control is a second-order system step response. Taking the inverse Laplace transform of equation (4), the time-domain expression of the DC current is given by equation (5):

[0073]

[0074] In the formula, t0 represents the injection control switching time; λ1 and λ2 are the characteristic roots of the closed-loop transfer function shown in formula (4); A1 to A3 are constant coefficients.

[0075] Figure 5 The following conditions are given for permanent faults with different transition resistances occurring at the midpoint of the line: dcI The simulated waveform. (From...) Figure 5 It can be seen that regardless of the value of the transition resistance, i dcI It can always track instruction values ​​quickly and effectively. It should be noted that... Figure 5 The slight oscillations in the initial waveform after medium current injection are caused by the high-frequency components of the traveling wave being reflected and deflected at the line port and fault point. Since the propagation process is accompanied by rapid attenuation of the traveling wave, to simplify the analysis... Figure 3 The circuit diagram shown does not consider the influence of wave processes. Through analysis of... Figure 5 Observations show that the transmission and attenuation of traveling waves do not affect i dcI Effective tracking of instruction values.

[0076] Voltage characteristic analysis:

[0077] according to Figure 3 It can be seen that, because i dcR =0, so u dcR equal to the fault point voltage u f Because i dcIpu Capable of quickly tracking step signal i dcref Therefore, u dcR It can be obtained from equation (6):

[0078] udcR = u f = i dcref R f (6)

[0079] Figure 6 gives the simulation waveforms of u when a permanent fault with different transition resistances occurs at the midpoint of the line dcR . Observing Figure 6 it can be seen that since the magnitudes of the injected current signals are the same, the DC voltage is proportional to the transition resistance. Considering when the maximum transition resistance is 500 Ω

[19] , according to Equation (6), u dcR = (1 × 2 × 500) kV = 500 kV, which is consistent with the blue line in the figure

[0080] Analysis of the characteristics of transient fault current:

[0081] Analysis of current characteristics: As Figure 7 shown is the equivalent circuit of the system under transient fault

[0082] In the Figure 7 circuit shown, i dcR = i dcI is always 0 and does not change with the change of the DC current command before and after t0. A transient fault means that in Equation (2), R eq → ∞, Figure 4 the denominator part of the red block diagram representing the actual circuit model in dcIpu is infinite, the original closed-loop control structure becomes open-loop, and i dcref cannot effectively track i

[0083] Figure 8 gives the simulation waveforms of i dcI under transient fault. Observing Figure 8 it can be seen that i dcI finally tends to 0, but it is not 0 but experiences a certain oscillation in the initial stage after the injection control switch. Also because Figure 7 the wave process is not considered. In fact, according to the principle of traveling wave protection of the line, if τ represents the time for the traveling wave to propagate along the full length of the line, then in the time scale of t0 < t < t0 + 2τ (the first traveling wave), the line model is equivalent to the wave impedance, as Figure 9 shown

[15] . In the figure, Z c represents the wave impedance of the line, and its expression is as shown in Equation (7):

[0084]

[0085] In the formula, R, L, C, and G are the resistance, inductance, conductance, and capacitance of the DC line per unit length respectively

[0086] Due to the very small line resistance and conductance, the wave impedance Z in the traveling wave stage is usually regarded as a resistance, and its value is c Therefore, in the initial stage after the injection control is switched, Equation (2) can be rewritten as: Therefore, in the initial stage after the injection control is switched, Equation (2) can be rewritten as:

[0087]

[0088] Combined with Figure 2 and Equation (8), it can be seen that at this time Figure 4 still remains in the closed-loop state, Figure 7 within t0 < t < t0 + 2τ in, the tracking process of i to i dcI constitutes the traveling wave injected by the MMC into the DC line. Then, the injected traveling wave reflects back and forth at both ends of the line, and the traveling wave propagation process is accompanied by attenuation, which is reflected as an amplitude-reducing oscillation of the waveform in the time domain dcref The tracking process of i to i

[20] .

[0089] Analysis of voltage characteristics:

[0090] From Figure 7 it can be seen that after the faulty branch disappears, u dcR = u dcI . According to Figure 2 , u dcI is determined by the output m dc of the PI controller and is obtained from the current deviation i dcref -i dcIpu . Through the analysis above, it can be seen that the physical model determines that the current deviation cannot be eliminated, and i dcref will be greater than i dcIpu , resulting in the forced saturation of the PI controller, and its output is clamped at the upper limit value. Considering that under instantaneous faults, the hybrid MMC needs to re-establish the voltage to restore the system, therefore, the upper limit of the PI output is set to 1 p.u., corresponding to m dc = 1, u dcR = u dcI = u MMC is equal to the normal operating voltage. Figure 10 shows the simulation waveforms of u dcR and u dcI under instantaneous faults. Observing Figure 10 it can also be seen that under instantaneous faults, u dcI tends to 800 kV.

[0091] Fault nature identification criterion and fault recovery strategy: By analyzing the controllability of the current signal, it is found that for instantaneous faults, the control of the hybrid MMC over i dcI is in a closed loop, i dcI is controllable, and u dcRProportional to the transition resistance. For permanent faults, the hybrid MMC affects i. dcI The control is open-loop, i dcI Unable to reach the preset command value, u dcR This is equal to the DC operating voltage. Given the asymmetry of the converters on both sides of the hybrid DC system line, corresponding criteria can be set on the rectifier side and the inverter side respectively. Its advantage is that system recovery does not depend on communication. When a transient fault occurs and the communication link is damaged, power supply can still be effectively restored, improving the reliability of fault adaptive recovery.

[0092] Criteria for identifying the nature of faults in hybrid MMC stations:

[0093] As analyzed above, under permanent fault i dcIpu Always able to quickly track i dcref Under transient faults, the injection control switching instant (t0) <t<t0+2τ),i dcIpu Able to track i dcref After t>t0+2τ, the amplitude decreases and tends to zero axis for reduced oscillation. Therefore, the fault nature identification criterion of the hybrid MMC station based on the DC modulation ratio can be constructed as shown in equation (9):

[0094]

[0095] In the formula, the DC modulation ratio m dc It can be obtained from the output of the PI loop in the control system, where T is the data window length of the identification criterion. Considering the speed of control adjustment, T = 5ms is taken; τ = l / v, where l is the line length and v is the traveling wave propagation speed, approximately the speed of light; m Δ To determine the threshold, considering factors such as control system tracking error and ripple reflection, we take m as the threshold value. Δ =0.85. Simultaneously, multi-point consecutive judgment is employed to ensure reliable identification of the fault nature; that is, during the period [t0+2τ, t0+2τ+T], if m dc If three consecutive sampling points satisfy equation (9), the fault is considered to have disappeared, and the hybrid MMC immediately switches back to the original DC voltage control. Otherwise, the fault is considered to have not disappeared, and preparations are made for the next injection or direct blocking of the converter.

[0096] Criteria for identifying the nature of faults at LCC stations:

[0097] As analyzed above, the current command i should be set appropriately. dcref This helps the LCC distinguish between the two types of faults. In the case of the system described in this paper, the normal operating voltage level is 800kV, meaning that under transient faults, u dcR It tends towards 800kV. Considering a maximum transition resistance of 500Ω, this means that under a permanent fault, u dcR Tends towards (500×i)dcref )kV. Therefore, if we take i dcref =1kA (i.e., 0.5pu), the threshold u can be set. Δ = (800 + 500 × 1) / 2kV = 650kV. At the LCC station, the fault is determined by whether the voltage can be established, as shown in equation (10):

[0098] u dcR (t)>u Δ (10)

[0099] Equation (10) can be used to determine the phase shift after the LCC phase shift process is completed: when u dcR If three consecutive sampling points satisfy equation (10), the fault is considered to have disappeared, and the LCC immediately switches back to the original DC current control. Otherwise, the fault is considered not to have disappeared, and the converter is directly blocked after the next injection or after a certain time limit.

[0100] System fault recovery strategy:

[0101] System recovery strategies such as Figure 11 As shown, the specific steps include:

[0102] Step 1: After receiving the line protection action signal, the LCC station performs a phase shift operation. After the phase shift process is completed, it collects u... dcR The data substitution formula (10) detects whether the voltage is rebuilt. After receiving the line protection action signal, the hybrid MMC station switches to DC current suppression mode (mode II). At the same time, the counter n=1 is set, which means that the first injection is about to be started.

[0103] Step 2: After 300ms of deionization, the hybrid MMC switches to current injection mode (mode III) and attempts to inject a step current signal with an amplitude of 0.5pu into the line.

[0104] Step 3: Acquire the m of the PI output during the time period from 2τ to 2τ+T after injection. dc Substitute the data into (9) to determine the nature of the fault. If m dc Equation (9) could not be satisfied within 3 consecutive sampling points, and n <n max (n max If the maximum number of injections is n, then let n = n + 1, the hybrid MMC switches to current suppression mode (mode II), and jumps to step two. If m dc Equation (9) could not be satisfied within 3 consecutive sampling points, and n = n max Then the hybrid MMC is locked. If m dc When three consecutive sampling points satisfy equation (9), the hybrid MMC immediately switches back to the original DC voltage control.

[0105] Step 4: When performing Steps 2 to 3, continuously judge whether u dcR satisfies Equation (10). Once Equation (10) is satisfied, the LCC immediately switches back to the original DC current control to restore the system current. If Equation (10) cannot be satisfied within the time period of (n max +1)×300 ms after the protection action, the LCC is blocked.

[0106] Example implementation: To verify the effectiveness of the proposed fault recovery strategy, a hybrid HVDC transmission system model as shown in Figure 1 is built in PSCAD. The main system parameters are shown in Appendix Table A1. The hybrid MMC adopts the nearest level approximation modulation. The DC line adopts the frequency-dependent model, and the tower structure is shown in Appendix Figure A1. The simulation step is 50 μs, and the sampling frequency is 10 kHz. Before the fault, the system operates under the rated conditions, and all faults occur at t = 1 s. The duration of all transient faults is 150 ms. Considering the fast operation requirement of the DC line protection, it is assumed that the protections on both sides of the line trip at t = 1.002 s after the fault occurs. At t = t0 = 1.302 s, the hybrid MMC switches from Mode II to Mode III and starts injecting current into the DC line.

[0107] Transient fault: Taking the solid fault as an example, Figure 12-14 For the Figure 1 permanent positive pole grounding fault occurring at f3, the corresponding simulation waveforms are given. The subscripts "p" and "n" in the legend represent the positive pole and the negative pole respectively.

[0108] In Figure 12 only the m dc waveform of the faulty pole is given. For the non-faulty pole, m dc is constantly 1 because the line protection does not operate. The sudden increase of m 0+ at time t dc in the figure is caused by excessive current deviation due to sampling and control delay. After that, within the time period of t0 < t < t0 + 2τ, under the closed-loop control of the current, m dc gradually decreases. When t = t0 + 2τ = 1.306 s, the collected m dc is substituted into Equation (9) for identification. After t > 1.306 s, the hybrid MMC begins to lose control of the DC current, and the current approaches the zero axis. However, due to the existence of distributed capacitance, the measured current shows a damped oscillation trend, and m dc oscillates upward, and the PI tends to saturate. All three fault cases first satisfy Equation (9) at t = 1.3076 s and continuously satisfy Equation (9) within the next 2 sampling points. According to the protection judgment logic, the criterion shown in Equation (9) can accurately and reliably identify transient faults. The hybrid MMC switches back to the DC voltage control at t = 1.3078 s. After t > 1.3078 s, mdc =1.

[0109] Figure 13 It can be seen that when t = 1.312s, u dcR Equation (10) is satisfied for the first time and continues to be satisfied for the next two sampling points. According to the protection judgment logic, the criterion shown in Equation (10) can accurately and reliably identify transient faults, and the LCC switches back to DC current control at t = 1.3122s. Afterwards, under control, the DC current gradually recovers to its pre-fault rated value. Combined with... Figure 13 and Figure 14 It can be seen that when t>1.5s, the system can be completely restored to the state before the fault.

[0110] Permanent failure: Taking metallicity as an example, Figure 15 and Figure 16 against Figure 1 The simulation waveforms for permanent bipolar ground faults occurring at points f1 to f3 are given. Since the electrical quantities are symmetrical under a bipolar fault, only the positive waveform is given here for ease of observation and analysis.

[0111] exist Figure 15 In the middle, m under three fault locations dc The waveform exhibits a clear segmented characteristic after t0, with the durations of the first segment being τ, τ / 2, and 0 (ignoring t). 0+ The spike pulse at the specified time corresponds precisely to the duration of the first traveling wave injected at each fault location, verifying the correctness of the analysis in Section 3. When t = t0 + 2τ = 1.306 s, the collected m... dc Substitute into equation (9) for identification. Until t = t0 + 2τ + T = 1.309s, m under the three fault locations... dc Equation (9) is never satisfied. According to the protection judgment logic, the criterion shown in Equation (9) can accurately and reliably identify permanent faults. The hybrid MMC switches back to DC current suppression control (mode II) at t = 1.309s and waits for the next injection or blockout according to the preset maximum number of injections.

[0112] Figure 16 It can be seen that u under the three fault locations dcR The waveforms are similar. Due to the short circuit at the fault point, the hybrid MMC cannot rebuild the voltage, u dcR If no voltage is detected, equation (10) is not satisfied. According to the protection judgment logic, the criterion shown in equation (10) can accurately and reliably identify permanent faults, and the LCC can reliably not restart.

[0113] Adaptability of criteria under different fault conditions: To comprehensively examine the adaptability of the proposed fault recovery strategy to fault type, fault distance, and transition resistance, simulations were performed under different fault conditions. Specific simulation results are shown in Tables 1 and 2. Table 1 simulates transient faults, with m... dc and u dcR The third consecutive sampled value greater than the threshold within the data window; t MMC and t LCC These represent the times when the hybrid MMC station and LCC station switch back to normal operation control strategies, respectively. Table 2 simulates permanent faults, m dc and u dcR Retrieve the maximum value within the data window.

[0114] Table 1. Simulation results under different transient fault conditions:

[0115]

[0116]

[0117] Table 2. Simulation results under different permanent fault conditions:

[0118] Fault type α Transition resistance <![CDATA[m dc ]]> <![CDATA[u dcR ]]> Identification results Positive grounding 0％ 500Ω 0.38 0kV Permanent Bipolar short circuit 20％ 400Ω 0.38 163kV Permanent Negative grounding 40％ 300Ω 0.53 235kV Permanent Positive grounding 60％ 200Ω 0.40 174kV Permanent Bipolar short circuit 80％ 100Ω 0.14 49kV Permanent Negative ground 100％ 0Ω 0.01 1kV Permanent

[0119] As shown in Table 1, for transient faults, since the equivalent circuit of the system remains the same after the fault disappears, the m value in the test results is... dc u dcR Similar values, not significantly affected by fault conditions, all close to 1. m under all transient fault conditions. dc All can satisfy equation (9) within 3 consecutive points, u dcR Both can satisfy equation (10) within three consecutive points. Therefore, it can be seen that the criteria shown in equations (9) and (10) can accurately and reliably identify transient faults. The converter quickly switches back to the normal operation control strategy, and the system can quickly restore power supply under the regulation of the converter control action.

[0120] As shown in Table 1, for permanent faults, under the same fault type, as the transition resistance increases and the fault distance decreases, u dcR It shows an increasing trend. From the test results, the measured m under all permanent failure conditions... dc The maximum value is 0.53, and m is always present. dc <m Δ ;u dcR The maximum value is 235kV, and u is always present. dcR Δ Therefore, it can be seen that the criteria shown in equations (9) and (10) can accurately and reliably identify permanent faults, and the system can reliably not recover. ​

[0121] In summary, the proposed fault recovery strategy can adapt to different fault conditions.

[0122] Impact of Noise: UHVDC transmission systems have high voltage levels and significant electromagnetic interference, inevitably leading to noise interference during on-site sampling. To examine the effectiveness of fault recovery strategies under noisy conditions, a scenario where the sampling device is severely affected by noise was considered: Gaussian noise with a signal-to-noise ratio of 30 dB was superimposed on the simulation data collected from the measurement points to simulate the actual sampling process. Tables 3 and 4 present the simulation results with 30 dB noise, traversing different fault conditions.

[0123] Table 3. Simulation results of transient faults under 30dB noise:

[0124] Fault type α Transition resistance <![CDATA[m dc ]]> <![CDATA[u dcR ]]> Identification results <![CDATA[t MMC / s]]> <![CDATA[t LCC / s]]> Positive grounding 0％ 500Ω 0.96 675kV Transient 1.3078s 1.3132s Bipolar short circuit 20％ 400Ω 0.95 675kV Transient 1.3077s 1.3132s Negative grounding 40％ 300Ω 0.96 672kV Transient 1.3077s 1.3131s Positive grounding 60％ 200Ω 0.95 676kV Transient 1.3078s 1.3132s Bipolar short circuit 80％ 100Ω 0.95 673kV Transient 1.3076s 1.3131s Negative grounding 100％ 0Ω 0.95 674kV Transient 1.3077s 1.3131s

[0125] Table 4. Simulation results of permanent faults under 30dB noise:

[0126] Fault type α Transition resistance <![CDATA[m dc ]]> <![CDATA[u dcR ]]> Identification results Positive grounding 0％ 500Ω 0.38 0kV Permanent Bipolar short circuit 20％ 400Ω 0.39 165kV Permanent Negative ground 40％ 300Ω 0.54 237kV Permanent Positive grounding 60％ 200Ω 0.40 176kV Permanent Bipolar short circuit 80％ 100Ω 0.15 50kV Permanent Negative grounding 100％ 0Ω 0.03 1kV Permanent

[0127] As can be seen from Tables 3 and 4, the proposed fault recovery strategy still has sufficient margin to ensure the correct identification of the fault nature under a noise intensity of 30dB, and can quickly restore power supply under transient faults.

[0128] In hybrid DC transmission systems using overhead lines as power transmission channels, DC line faults are inevitable, necessitating safe, fast, and reliable fault recovery strategies. This paper addresses the lack of fault nature discrimination in existing recovery strategies. By fully utilizing the control characteristics of hybrid MMC (Multi-Mode Controlled Transmission) systems, aiming to improve system restart success rate and reduce secondary impacts, a step current is injected into the line after deionization to reflect the system's dynamic characteristics under different fault types. Therefore, a DC line fault recovery strategy based on the controllability of DC current is proposed. The main contributions are as follows:

[0129] 1) The use of current as the controlled variable effectively prevents overcurrent surges caused by characteristic signal injection under converter outlet faults.

[0130] 2) The response characteristics of electrical quantities after signal injection were analyzed based on the equivalent circuit of the system under different fault types: For transient faults, the inverter side current is uncontrollable and the rectifier side voltage is the system voltage; for permanent faults, the inverter side current is controllable and the rectifier side voltage is proportional to the transition resistance.

[0131] 3) Using the control signal output by the PI as a characteristic quantity, a fault characteristic identification criterion based on the DC voltage modulation ratio was constructed on the inverter side.

[0132] 4) Extensive simulation results demonstrate that the proposed fault recovery strategy can adapt to various fault conditions occurring throughout the entire line, withstand transition resistance up to 500Ω, tolerate noise with a signal-to-noise ratio of 30dB, and is insensitive to changes in system parameters, exhibiting good versatility. Furthermore, the fault recovery strategy does not rely on a communication system and has low requirements for the sampling rate of the protection device, making it easy to implement in engineering applications.

[0133] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0134] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A fault recovery method for ultra-high voltage hybrid lines based on controllable DC current, characterized in that, Includes the following steps: Step 1: After receiving the line protection action signal, the LCC station performs a phase shift operation. After the phase shift process is completed, it collects the voltage at the protection installation point at the beginning of the line. Data substitution formula (10) To determine if the voltage can be rebuilt. To set the threshold, the hybrid MMC station switches to DC current suppression mode after receiving a line protection action signal, and simultaneously sets a counter. This indicates that the first injection is about to begin; Step 2: After 300ms of deionization, the hybrid MMC switches to current injection mode and attempts to inject a step current signal with an amplitude of 0.5pu into the line. Step 3: Collect PI output data during the period from 2t to 2t+T after injection. Substitute the data into (9) to determine the nature of the fault. Equation (9) could not be satisfied within 3 consecutive sampling points. In the formula, DC modulation ratio The data is obtained from the output of the PI circuit in the control system, where T is the data window length for the identification criterion. , For line length, Let be the speed of traveling wave propagation, and be the speed of light. To determine the threshold, take ;and , To maximize the number of injections, let The hybrid MMC switches to current suppression mode and proceeds to step two. Equation (9) could not be satisfied within 3 consecutive sampling points, and Then lock out hybrid MMC, if m dc When three consecutive sampling points satisfy equation (9), the hybrid MMC immediately switches back to the original DC voltage control; Step 4: Real-time judgment during steps 2 and 3. u dcR If equation (10) is satisfied, the LCC immediately switches back to the original DC current control to restore the system current. If this occurs after the protection operation... If equation (10) cannot be satisfied within the ms time period, then the LCC is blocked.

2. The fault recovery method for ultra-high voltage hybrid lines based on controllable DC current according to claim 1, characterized in that, Based on an ultra-high voltage hybrid direct current (UHVDC) transmission system, each pole of the system adopts a dual-valve group structure with symmetrical parameters. On the rectifier side, LCC converters (i.e., dual 12-pulse series connections) are used, while on the inverter side, a hybrid MMC converter (full / half-bridge submodule) is employed. During normal system operation, the LCC station uses constant DC current control, and the MMC station uses constant DC voltage / reactive power control. The voltage and current at the line-end protection installation are denoted as follows: , The voltage and current at the protection installation point at the end of the line are respectively denoted as... , .

3. The fault recovery method for ultra-high voltage hybrid lines based on controllable DC current according to claim 2, characterized in that, For LCC substations, when a DC line fault occurs, the DC line protection should operate rapidly. For LCC substations, after receiving the line protection operation signal, the pole control system urgently shifts the thyristor firing angle to 120°, causing the LCC to switch from rectification mode to inverter operation. This allows the fault energy in the DC system to be fed back to the AC grid, reducing the current flowing into the fault point from the LCC. ,when When the trigger angle is reduced to near zero, the trigger angle is further phase-shifted to over 150°.

4. The fault recovery method for ultra-high voltage hybrid lines based on controllable DC current according to claim 2, characterized in that, For LCC substations and MMC substations, active control of fault current is achieved by switching control modes. Thanks to the negative voltage output capability of the full-bridge submodule, the DC voltage of the hybrid MMC has a certain adjustment margin. On the one hand, during DC fault ride-through, this adjustment margin is used to adjust the DC voltage to suppress the fault current output to the short-circuit point. On the other hand, during DC fault recovery, this adjustment margin is used to inject a small-amplitude characteristic signal into the DC line to help determine whether the fault point has disappeared.

5. The fault recovery method for ultra-high voltage hybrid lines based on controllable DC current according to claim 4, characterized in that, It also includes the analysis of electrical quantity response characteristics under different fault types, specifically including the electrical quantity analysis of permanent faults and the characteristic analysis of transient fault currents.

6. The fault recovery method for ultra-high voltage hybrid lines based on DC current controllability according to claim 5, characterized in that, The electrical quantity analysis of the permanent fault includes current characteristic analysis and voltage characteristic analysis.

7. The fault recovery method for ultra-high voltage hybrid lines based on DC current controllability according to claim 5, characterized in that, The transient fault current characteristic analysis includes current characteristic analysis and voltage characteristic analysis.

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