Hybrid current limiting control method for half-bridge MMC direct current side fault

Abstract

The invention relates to the technical field of power system protection, in particular to a hybrid current-limiting control method for a half-bridge MMC direct-current side fault, and the method comprises the steps: firstly inputting control current limiting, and then inputting equipment current limiting, thereby achieving the hybrid current-limiting control, and restraining the rapidly-increasing fault current. When the current limiting effect is controlled to be maximum, equipment current limiting is started, loop impedance is increased, direct-current side fault current is further suppressed, and current limiting is controlled to adaptively and dynamically increase direct-current side voltage so as to reduce alternating-current side current amplitude; in the period of controlling the current limiting effect, the bridge arm current rises, and after the current limiting effect of the device, the bridge arm current is effectively restrained, it is ensured that the IGBT current is in the safety interval of the device, and potential safety hazards caused by direct current and bridge arm overcurrent to related devices are reduced. According to the method, the starting control logic of the control current limiting strategy and the equipment current limiting strategy is clear, the monitoring and self-adaptive control execution logic is simple, the operation controllability and the strategy usability are well considered, and engineering application and popularization are facilitated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system protection, and particularly relates to a hybrid current limiting control method for half-bridge MMC DC side faults. BACKGROUND

[0002] Modular Multi-level Converter based on High Voltage Direct Current (MMC-HVDC) power grid has the advantages of fast decoupling of active power and reactive power, no commutation failure, etc., and is considered as one of the key technologies for solving the problem of high proportion of renewable energy grid connection in the future.

[0003] The MMC system using half-bridge sub-modules (HBSM) has become the mainstream of the current MMC-HVDC power grid due to its cost advantage. The half-bridge modular multilevel converter (half-bridge MMC) is composed of a plurality of half-bridge sub-modules, each of which includes two insulated gate bipolar transistors (IGBT) and a DC supporting capacitor. The half-bridge sub-module realizes multi-level voltage output by the input and output of the sub-module, has the advantages of low harmonic content and flexible control, and is widely used in new energy grid connection, high voltage direct current transmission and other fields. However, this structure does not have the ability to clear faults, and must rely on DC circuit breakers (DCCB) to cut off the fault line to achieve fault clearing and recovery. The half-bridge MMC DC side fault mainly refers to the short circuit (such as metallic short circuit, high resistance short circuit, etc.) of the DC line, which causes the rapid discharge of the equivalent capacitor of the converter, resulting in a sharp rise of the DC side fault current and the bridge arm current. The equivalent impedance of the fault loop is small, the DC fault current rises rapidly, and the peak value is large. Not only does it require a very high breaking capacity of the DC circuit breaker, but it may also cause damage to the core devices such as IGBT due to overcurrent, resulting in system-level safety risks. By limiting the growth rate and peak value of the fault current through current limiting technology, the high breaking capacity requirement of the circuit breaker can be significantly reduced, which is one of the necessary means to solve the problem.

[0004] The existing DC fault current limiting methods can be divided into control current limiting and device current limiting. The essence of control current limiting is to reduce the equivalent discharge source of MMC to reduce the fault current growth rate and peak value to achieve current limiting. Device current limiting changes the system impedance loop by using series resistors, inductors, capacitors and power electronic current limiters to suppress the fault current rise. However, in the existing current limiting methods, the main method is to simply control the current limiting, which lacks reasonable constraints on the adjustment parameters, making it difficult to balance the current limiting effect and device safety. The simple device current limiting mainly uses fixed input method, which cannot adapt to different degrees of fault. Moreover, there is a lack of coordination between control current limiting and device current limiting, resulting in overcurrent risk of DC and bridge arm. SUMMARY

[0005] In order to solve the above technical problems, the application adopts the following technical solutions:

[0006] In order to solve the above technical problems, the application adopts the following technical solutions:

[0007] A hybrid current limiting control method for half-bridge MMC DC side fault, the converter valve of the MMC includes a, b, c three-phase, each phase includes an upper bridge arm and a lower bridge arm; each bridge arm includes a plurality of series-connected sub-modules, each sub-module is a half-bridge sub-module; each bridge arm of each phase is also respectively connected in series with a plurality of damping modules for implementing current limiting and controllable input state, the initial state of each damping module is in the state of bypass input;

[0008] The method comprises the following steps:

[0009] S1: Real-time acquisition and monitoring of the bridge arm current of the MMC, preliminary judgment of whether the DC line has a fault; when it is judged that the DC line has a fault, step S2 is executed;

[0010] S2: Start the control current limiting strategy, dynamically control the number of sub-modules input in the single-phase bridge arm by tracking the size of the bridge arm current, to suppress the fault current in the DC line, and analyze and determine the upper limit of the control current limiting strategy, when the control current limiting strategy reaches the upper limit, step S3 is executed;

[0011] S3: Start the device current limiting strategy, by tracking the size of the bridge arm current amplitude, adaptively control the number of damping modules input in the bridge arm, to further control the fault current in the DC line.

[0012] As a preferred scheme, in step S1, the bridge arm current change rate is used to preliminarily judge whether the DC line has a fault; the condition for judging that the DC line has a fault is:

[0013]

[0014] Wherein, |d ij / dt| is the absolute value of the bridge arm current change rate; |d ij / dt| max is the peak value of the bridge arm current change rate in the steady state; k1 is a preset reliability coefficient.

[0015] As a preferred scheme, in step S2, after starting the control current limiting strategy, the number of sub-modules input in the single-phase bridge arm is dynamically controlled according to the set sub-module bypass proportion coefficient k:

[0016]

[0017] wherein n is the number of sub-modules put into during the linear line fault of the MMC single phase, and is a dynamic control variable; N is the number of sub-modules put into under normal circumstances for maintaining the stability of the DC side voltage, and is a constant value.

[0018] As a preferred solution, in step S2, the specific way of dynamically controlling the number of sub-modules put into in the single phase bridge arm is:

[0019] At the start of the control of the current limiting strategy, the initial value of the sub-module bypass proportion coefficient k is set as k = 1-m AC , m AC is the AC modulation ratio of the half-bridge MMC; then, by tracking the size of the bridge arm current, the value of the sub-module bypass proportion coefficient k is determined adaptively by using the following current limiting suppression curve:

[0020] k = a · i ij +b, a = 2.5(m AC +k max -1), b = 3.75-3.75m AC -2.75k max ;

[0021] wherein a and b are the parameters of the current limiting suppression curve; i ij is the size of the bridge arm current; k max is the upper limit value of the sub-module bypass proportion coefficient k;

[0022] In the process of controlling the current limiting strategy, after the value of the sub-module bypass proportion coefficient k is determined, the number of sub-modules put into n is determined according to the formula ; when the value of the sub-module bypass proportion coefficient k reaches the upper limit value k max , the control of the current limiting strategy reaches the upper limit of current limiting.

[0023] As a preferred solution, the upper limit value k max of the sub-module bypass proportion coefficient k is determined as follows:

[0024] When the AC side of the half-bridge MMC is connected to the strong AC power grid, the upper limit value k max of the sub-module bypass proportion coefficient k is 0.4-0.8;

[0025] When the AC side of the half-bridge MMC is connected to the weak AC power grid, the upper limit value k max of the sub-module bypass proportion coefficient k is 1-m AC -0.4.

[0026] As a preferred embodiment, for any l-th damping module, including the damping resistor R ADl and the damping resistor R ADl Parallel switching devices T ADl In the initial state, the switching device T ADl When in the conducting state, the damping resistor R ADl When bypassed, the l-th damping module is in the bypassed state; when the switching device T ADl When the control is disconnected, the damping resistor R ADl In the bridge arm where the series value is located, the l-th damping module is put into operation.

[0027] As a preferred embodiment, the number of damping modules connected in series in each arm of each phase is d, and the damping resistance value of each damping module is equal, and the sum of the damping resistances of the d damping modules is R. AD satisfy:

[0028]

[0029] L eq C eq These represent the equivalent inductance and equivalent capacitance of the equivalent second-order RLC series circuit of the fault circuit, respectively.

[0030] As a preferred embodiment, in step S3, the specific method for adaptively controlling the number of damping modules in the bridge arm by tracking the magnitude of the bridge arm current is as follows:

[0031] Define d interval factors r l l = 1, 2, ..., d, and 1.5 = r1 < r2 < ... < r d <2, based on the rated amplitude i of the bridge arm current armmax , the interval [1.5i armmax ,2i armmax The bridge arm current amplitude is divided into d sub-intervals, as follows:

[0032] [r1i armmax ,r2i armmax ),

[0033] [r2i armmax ,r3i armmax ),

[0034] ...

[0035] [r d i armmax ,2i armmax ];

[0036] Then, by tracking the amplitude i of the bridge arm current arm The magnitude of the bridge arm current iarm the value of the interval factor falls within the sub-interval of the bridge arm current amplitude with r l as the lower limit, the number of the damping modules put into the bridge arm is l.

[0037] As a preferred solution, the number of the damping modules in series in each bridge arm of each phase is 4, and the values of the set 4 interval factors are respectively:

[0038] r1=1.5, r2=1.6, r3=1.7, r4=1.8.

[0039] As a preferred solution, after the device current limiting strategy is started, the size of the bridge arm current is continuously tracked in the current limiting strategy, and with the decrease of the bridge arm current, the value of the sub-module bypass proportion coefficient k gradually decreases from the upper limit value k max , so as to dynamically control the number of the sub-modules put into the single-phase bridge arm.

[0040] Compared with the prior art, the present application has the following beneficial effects:

[0041] 1. The hybrid current limiting control method for the half-bridge type MMC DC side fault proposed in the present application realizes hybrid current limiting control by first putting in control current limiting and then putting in device current limiting, so as to suppress the rapidly growing fault current; the device current limiting is started when the control current limiting effect is the largest, the loop impedance is increased, the DC side fault current is further suppressed, and the control current limiting adaptively and dynamically lifts the DC side voltage to reduce the AC side current amplitude; during the control current limiting action, the bridge arm current is lifted, and after the device current limiting action, the bridge arm current is effectively suppressed, so as to ensure that the IGBT current is in the device safety interval and reduce the safety hidden danger of the DC and bridge arm overcurrent to the related devices.

[0042] 2. In the method of the present application, the sub-module bypass proportion k is rapidly adjusted to 1-m AC in the preliminary current limiting stage of the control current limiting, so as to realize the preliminary suppression of the bridge arm current, the value of k is dynamically adjusted based on the linear current limiting suppression curve in the dynamic current limiting stage of the control current limiting, and the device current limiting cooperates, so as to quickly respond to the current surge in the initial stage of the fault, and accurately adapt the current limiting strength according to the current change, compared with the pure control current limiting or device current limiting, the DC side fault current and the bridge arm current peak value can be significantly reduced, and the risk of equipment overcurrent damage can be reduced.

[0043] 3. In the method of the present application, the device current limiting is started when the control current limiting reaches the upper limit, i.e. k=k max , and the control current limiting will be reset after the device current limiting is started, i.e. the value of the sub-module bypass proportion k is k maxGradually reduce; in this process, both the quick response characteristics of the control current limiting and the influence of the AC side current on the bridge arm current are suppressed by the device current limiting, and the problem of insufficient current limiting effect caused by the lack of cooperation in the prior art is solved.

[0044] 4、The starting control logic of the control current limiting strategy and the device current limiting strategy of the method is clear, and the monitoring and adaptive control execution logic is simple, which well balances the controllability and strategy usability, and is conducive to engineering application promotion. BRIEF DESCRIPTION OF DRAWINGS

[0045] The accompanying drawings, which form a part of this specification, are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification. The embodiments of these drawings are set forth to explain the application and are not limiting of the application. It should be understood that these drawings are not necessarily to scale and that, in certain instances, various aspects of the application can be shown exaggerated or minimized for the purposes of illustration.

[0046] Figure 1 It is a schematic diagram of MMC-HVDC system topology.

[0047] Figure 2 It is a principle diagram of the hybrid current limiting control method for the half-bridge MMC DC side fault of the application.

[0048] Figure 3 It is a schematic diagram of the optimized MMC-HVDC system topology based on the method of the application.

[0049] Figure 4 It is a schematic diagram of the current limiting suppression curve in the control current limiting strategy of the method of the application.

[0050] Figure 5 It is an equivalent circuit diagram of MMC AC side.

[0051] Figure 6 It is a principle diagram of the adaptive control of the number of damping modules in the bridge arm in the device current limiting strategy of the method of the application.

[0052] Figure 7 It is an MMC bridge arm current effect diagram of the hybrid current limiting method of the application.

[0053] Figure 8 It is a DC side current and bridge arm current waveform diagram after fault in different cases in the embodiment. DETAILED DESCRIPTION

[0054] For the purposes of making the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0055] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0056] In view of the problems in the prior art, the present application provides a hybrid current limiting control method for half-bridge MMC DC side fault, the method of the present application takes a half-bridge MMC-HVDC system as the research object, and the framework structure is as shown in Figure 1 The converter valve of the MMC includes three phases of a, b and c, each phase includes an upper bridge arm and a lower bridge arm; each bridge arm includes a plurality of series-connected sub-modules SM, and each sub-module SM is a half-bridge sub-module. The working state of the half-bridge sub-module has three types, namely, an input state, a bypass state and a lockout state. After a short-circuit fault occurs in the DC line, the fault current rapidly increases, and the IGBT will be broken down and damaged due to the rise in junction temperature within milliseconds. The temperature is not convenient for high-speed collection, and in actual engineering, the 2 times rated current is taken as the overcurrent threshold to start the IGBT self-protection to be unlocked.

[0057] The bridge arm current is composed of the AC side and DC side current components, and taking the phase A as an example, the upper and lower bridge arm currents are as shown in formula (1).

[0058]

[0059] In the formula, i dc and i ac are the AC side and DC side currents during the fault, and under the DC side fault control current limiting, the bridge arm IGBT in the converter station connected with the strong AC side still has a high possibility of overcurrent lockout due to the high rise of the AC side current.

[0060] The technical idea of the hybrid current limiting control method for the half-bridge MMC DC side fault is to first input control current limiting and then input equipment current limiting to suppress the rapidly growing fault current and prevent IGBT lockout, as shown in the principle of Figure 2 When the control current limiting effect is the largest, the equipment current limiting is started, the loop impedance is increased, the DC side fault current is further suppressed, and the control current limiting adaptively dynamically raises the DC side voltage to reduce the AC side current amplitude. During the control current limiting action, the bridge arm current is raised, and after the equipment current limiting action, the bridge arm current is effectively suppressed to ensure that the IGBT current is in the equipment safety interval.

[0061] Based on this technical idea, a current limiting damping module for implementing the equipment current limiting strategy is added. As shown in Figure 3 In the present application, a plurality of damping modules SM AD for implementing current limiting and having controllable input state are further added in each bridge arm of each phase of the half-bridge MMC, as shown in AD The initial state of each damping module SM ij is in the bypass input state, waiting for control input in the equipment current limiting control stage to further suppress the DC side fault current.

[0062] According to the above technical idea, the hybrid current limiting control method for the half-bridge MMC DC side fault proposed by the present application specifically includes the following steps:

[0063] S1: Real-time acquisition and monitoring of the bridge arm current of the MMC, preliminary judgment of whether the DC line has a fault; when it is judged that the DC line has a fault, step S2 is executed.

[0064] In this step, the bridge arm current change rate is used as the criterion for starting control current limiting to preliminarily judge whether the DC line has a fault. In specific implementation, the condition for judging that the DC line has a fault is:

[0065]

[0066] In the formula, |d ij / dt| is the absolute value of the bridge arm current change rate; |d ij / dt| max is the peak value of the bridge arm current change rate in the steady state; k1 is a preset reliability coefficient, which can be taken as 1.2 in specific implementation and application.

[0067] After the DC side fault, the fault current increases rapidly, which will increase the bridge arm current, so the bridge arm current change rate can be used as the criterion for starting control current limiting to enter step S2 to execute the control current limiting strategy.

[0068] S2: Activate the control current limiting strategy. By tracking the magnitude of the bridge arm current, dynamically control the number of sub-modules connected in a single-phase bridge arm to suppress the fault current in the DC line, and analyze and determine the upper limit of the control current limiting strategy. When the control current limiting strategy reaches the upper limit of the current limiting strategy, execute step S3.

[0069] In this step, after initiating current limiting control, the collected bridge arm current information directly controls the switching of bridge arm submodules in the valve control, eliminating transmission delay and enabling rapid initiation of current limiting. To effectively control the number of submodules activated in a single-phase bridge arm, this invention defines a submodule bypass ratio coefficient k, the relationship between which and the number of activated submodules n can be expressed as:

[0070]

[0071] Where n represents the number of sub-modules activated in a single phase of the MMC during a straight-line fault, and is a dynamically controlled variable; N represents the number of sub-modules activated to maintain a stable DC-side voltage, and is a constant value. By increasing the sub-module bypass ratio coefficient k, the number of sub-modules activated in a single phase of the MMC can be dynamically adjusted, thereby reducing the MMC output voltage and achieving a current-limiting effect on the fault current.

[0072] Specifically, when the current limiting strategy is activated, the initial value of the submodule bypass ratio coefficient k is set to k = 1 - m. AC m AC The AC modulation ratio of the half-bridge MMC is given; then, by tracking the magnitude of the bridge arm current, the value of the operator module bypass proportional coefficient k is determined using the following current limiting suppression curve adaptive metric:

[0073] k = a·i ij +b (4)

[0074] Where a and b are the parameters of the current limiting suppression curve; i ij This represents the magnitude of the bridge arm current.

[0075] In the current limiting strategy of this invention, the current control range of the current limiting suppression curve is defined as 1.1 times the rated current of the bridge arm to 1.5 times the rated current of the bridge arm, and the corresponding submodule bypass ratio coefficient k is 1-m. AC To k max Interval, k max This represents the upper limit of the submodule bypass ratio coefficient k. Therefore, in the current limiting suppression curve, there exists a value with [1.1i] as the upper limit. ijmax ,1-m AC ] and [1.5i ijmax k max The control inflection point, such as Figure 4 As shown. Based on the current limiting and suppression curve, the following relationship can be obtained:

[0076] 1-m AC =1.1a+b,k max =1.5a+b;

[0077] Based on the above relationship, the current limiting suppression curve parameters a, b can be solved as:

[0078] a=2.5(m AC +k max -1), b=3.75-3.75m AC -2.75k max ;

[0079] Wherein, a, b are current limiting suppression curve parameters; i ij is the size of the bridge arm current; k max is the upper limit value of the sub-module bypass proportion coefficient k.

[0080] The fault loop is composed of the equivalent impedance of MMC, the current limiting reactor and the line parameters in series, as shown in Figure 2 , which can be equivalent to a second-order RLC series circuit, and the fault current is shown in equation (5).

[0081] i dc (t)=Ae -σt sin(ω r t+β) (5)

[0082]

[0083] In the formula: i load is the current under the steady-state operation of MMC, L arm is the bridge arm inductance, R0 is the bridge arm resistance, R on is the on-resistance of the sub-module, C SM is the sub-module capacitance value, L dc is the current limiting reactor, R line and L line are the equivalent resistance and inductance of the line, L eq , R eq and C eq are the equivalent inductance, resistance and capacitance of the fault loop. σ is the time constant of current decay, ω dc is the inherent angular frequency of the fault loop, A is the fault current amplitude coefficient, ω r is the fault current angular frequency, and β is the initial phase angle of the fault current.

[0084] Under the control of the current limiting response, the voltage source of the fault loop is reduced from the rated value U dcN to u dc , and the equivalent capacitance value C eq1 of MMC changes, which is affected by the value of k and the unit capacitance constant T CThe common influence of the value. With the increase of bypass proportion coefficient k value, the equivalent capacitance C eq1 value increases significantly, T C value is proportional to C eq1 . Under the control of current limiting, the change of AC side voltage u dc the change of C eq1 , the increase of bypass proportion coefficient k value leads to the decrease of n value, and the influence of C eq1 is far less than u dc lower the inhibition of fault current i dc , so it can be considered that the change of C eq1 The influence of fault current is small and can be ignored.

[0085] The proportion of half-bridge MMC bypass sub-module k value increases, the number of sub-modules n value decreases, the DC outlet voltage decreases, and the AC side voltage decreases in proportion. From the AC side, it is equivalent to a three-phase symmetrical fault with different severity, as shown in Figure 5 , the AC fault current will help to increase the bridge arm current. The increase of k value in current limiting control will lead to the drop of AC voltage, and the drop degree is related to k value, as shown in formula (8):

[0086] ΔU sm = 0.5kU dc m AC (8)

[0087] When the half-bridge MMC is connected to the AC power grid on the AC side, the AC fault steady-state current is calculated by ΔU sm , as shown in formula (9), and the corresponding total current is shown in formula (10):

[0088]

[0089] In the formula: R ac and L ac are the equivalent resistance and reactance of MMC side AC side, σ ac is the time constant of AC side fault loop.

[0090] In the three-phase symmetrical fault of AC, under the joint action of fault current periodic component and decay component, the peak value of fault current appears at the moment of half cycle after fault, that is, 10ms

[25] after fault. The peak value of fault current is also called impact current, which can be calculated by fault steady-state, as shown in formula (11).

[0091]

[0092] According to formula (1), the changes in the arm currents, which are composed of nonlinearly varying AC and DC side currents, also exhibit nonlinearity, making it difficult to analyze the relationship between the arm currents and the AC and DC currents. Linearization is required. Different k values ​​can be set... max After a fault, the value of k jumps directly from order 0 to k. max At this time u dc and C eq1 It can be considered a constant value. The fault circuit is generally underdamped. During the initial capacitor discharge period of the fault, the DC side fault current can be considered to increase linearly. At the same time, the AC side current also increases linearly.

[0093] Based on the above analysis, it can be seen that after a fault, the fault current on both the AC and DC sides increases linearly, and the bridge arm current also increases linearly, as shown in equation (12).

[0094] i arm =k1i ac +k2i dc (12)

[0095] In the formula: k1 and k2 are the slopes corresponding to the linearized AC and DC currents, respectively.

[0096] After the fault, k is controlled in the current limiting process. max It is the most critical variable; analyzing different k max The effect of k value on the DC side fault current limiting effect varies with the current. max With the increase of k, the suppression effect of DC fault current gradually improves, and the demand for the breaking capacity of DC circuit breakers gradually decreases, indicating that k max The larger the value, the stronger the ability of the current limiting control to suppress DC fault current.

[0097] The current limiting control and equipment current limiting described in this invention both suppress fault current on the DC side. To prevent overcurrent blocking due to excessive increase in AC side current, it is necessary to ensure that the DC side current accounts for more than 50% of the bridge arm current. That is, considering the IGBT overcurrent stress, the parameter k in the current limiting control... max The lower limit is set to 0.4; and to prevent the AC side from disconnecting from the grid, k max 0.8 is an option.

[0098] Therefore, when the AC side of a half-bridge MMC is connected to a strong AC power grid, the upper limit of the submodule bypass ratio coefficient k is k. max The value ranges from 0.4 to 0.8.

[0099] When the AC side of the half-bridge MMC is connected to the AC weak grid, taking the wind farm weak grid as an example, the fault current provided by the wind farm is affected by the national standard, as shown in Equation (13). Due to the limitation of the inverter capacity, the fault current provided by the wind farm will not increase the bridge arm current, that is, there is no need for equipment current limiting.

[0100]

[0101] where i d and i q are the d-axis and q-axis current references, respectively, whose peak value ranges between 1.2-1.5 p.u., i d0 is the pre-fault d-axis current reference, ξ is the damping ratio of the second order system, ω n is the natural oscillation angular frequency, ω d is the damped oscillation frequency, β1 is the damping angle, and θ φ is the current initial phase angle.

[0102] Considering both current and voltage factors, the constraint boundary of k value in the control current limiting strategy is given, the lower limit of k is zero, and the upper limit of k is k max = 0.4. But since the initial value of the sub-module bypass proportional coefficient k is k = 1-m AC after the control current limiting strategy is started, the lower limit of k max cannot be lower than 1-m AC .

[0103] Therefore, when the AC side of the half-bridge MMC is connected to an AC weak grid, the upper limit value k max of the sub-module bypass proportional coefficient k is 1-m AC ~ 0.4.

[0104] In the control current limiting strategy of step S2, when the value of the sub-module bypass proportional coefficient k is determined, the number of single-phase input sub-modules n is determined according to the formula and the control current limiting strategy reaches the upper limit of current limiting when the value of the sub-module bypass proportional coefficient k reaches the upper limit value k max .

[0105] S3: Start the equipment current limiting strategy, and further control the fault current in the DC line by adaptively controlling the number of damping modules input in the bridge arm according to the size of the bridge arm current amplitude.

[0106] The effect of the control current limiting strategy of step S2 generally reaches the maximum after 1-2 ms of the fault, that is, k = k max , and the control current limiting strategy reaches the upper limit of current limiting when the value of the sub-module bypass proportional coefficient k reaches the upper limit value k max , and then the equipment current limiting strategy of step S3 is started to further suppress the fault current on the DC side.

[0107] Meanwhile, after the equipment current limiting strategy is started, the size of the bridge arm current is continuously tracked in the control current limiting strategy, and the value of the sub-module bypass proportional coefficient k decreases from the upper limit value k max .Gradually reduce, thereby dynamically control the number of sub-modules put in single-phase bridge arm; AC side current is reduced, eliminating the AC side fault current on the bridge arm to increase the impact of ensuring the bridge arm current to maintain in the safe interval.

[0108] Specific implementation, as Figure 3 shown, for any l damping module, including damping resistance R ADl And the damping resistance R ADl Parallel switch T ADl ; initial state, switch T ADl In the on state, so that the damping resistance R ADl Bypass, the l damping module is bypassed into the state; when the switch T ADl Control is disconnected, the damping resistance R ADl Access to the bridge arm in series, the l damping module into the state.

[0109] From the number of settings, set the number of damping modules in series in each bridge arm of each phase to d, the damping resistance value of each damping module is equal, then the damping resistance of d damping modules R AD Must satisfy the following formula:

[0110]

[0111] L eq , C eq Indicate the equivalent inductance and equivalent capacitance of the equivalent second-order RLC series circuit of the fault loop.

[0112] In the proposed hybrid current limiting, the value of k will change the value of C eq , when k max = 0.4, C eq1 = [C SM / N, C SM / (1-k max )N], formula (14) can be written as formula (15):

[0113]

[0114] When the fault loop is in the over-damped state, formula (5) can be written as formula (16), and the corresponding fault steady-state current is as formula (18).

[0115]

[0116] At this time, the DC fault current is mainly low-frequency component, which satisfies: R ADmin >> L eq At the same time, the AC side current returns to the state before the fault, and the bridge arm current amplitude i armwhich can be expressed as (19) and its amplitude should be less than 2 p.u.

[0117]

[0118] The power loss and absorbed energy generated by the bridge-arm damping module are:

[0119]

[0120] W AD =∫P AD (21)

[0121] Considering the randomness of fault location and transition resistance, the one-time investment of all damping modules has the problems of high current limiting cost and high heating of damping modules. Therefore, the sum of bridge-arm damping modules R AD is split into d R ADl modules in series. In the case of high resistance fault, only a small number of R ADl modules need to be put into operation, reducing the current limiting cost of each device.

[0122] Under this strategy, a step function is introduced to track the size of the bridge-arm current amplitude, and the number of damping modules put into the bridge-arm is adaptively controlled. The schematic diagram is shown in Figure 6 , and the specific way is:

[0123] Set d interval factors r l , l = 1, 2, …, d, and 1.5 = r1< r2< … < r d < 2, according to the rated amplitude of the bridge-arm current i armmax , the interval [1.5i armmax , 2i armmax ] is divided into d bridge-arm current amplitude sub-intervals, which are respectively:

[0124] [r1i armmax , r2i armmax ),

[0125] [r2i armmax , r3i armmax ),

[0126] …

[0127] [r d i armmax , 2i armmax ];

[0128] Then, by tracking the size of the bridge-arm current amplitude i arm , if the value of the bridge-arm current amplitude i arm falls into the interval with r l ,If the subinterval of the lower limit of the bridge arm current amplitude is taken, the number of the damping modules inserted in the bridge arm is 1.

[0129] In practical applications, if the value of d is too large, it will lead to the adaptive insertion of R ADl Complicated operation. Therefore, in comprehensive consideration, as a preferred mode, the number of the damping modules in series in each bridge arm of each phase is 4, at this time, the values of the set 4 interval factors can be respectively: r1=1.5, r2=1.6, r3=1.7, r4=1.8.

[0130] The mixed current limiting MMC bridge arm current effect diagram adopting the method of the application is shown in Fig. 2. Figure 7 As shown in the figure, during the control of the current limiting effect, the bridge arm current is lifted, and after the device current limiting effect, the bridge arm current is effectively suppressed, ensuring that the IGBT current is in the device safety interval.

[0131] Embodiment:

[0132] To verify the technicality of the scheme of the application, the application is further analyzed and explained below in combination with the embodiment and experimental data.

[0133] In this embodiment, a true bipolar MMC-HVDC system is taken as the research object, the submodules in the MMC converter station adopt the equivalent model, the MMC2 converter station is controlled at a constant active power, and the MMC1 converter station is controlled at a constant DC voltage. The bridge arm current sampling frequency is selected as 100 kHz, it is assumed that a fault occurs at the DC side at t=0 ms, the fault diagnosis is completed at t=3 ms, and the trip command is given to the DC circuit breaker, and the action time of the DC circuit breaker is selected as 2 ms, that is, the mixed current limiting needs to ensure that the bridge arm current does not exceed the limit within 5 ms after the fault, and the DC side fault current is suppressed as much as possible; the DC system parameters are shown in Table 1:

[0134] Table 1: MMC-HVDC system parameters

[0135]

[0136] The most extreme case is considered, and the metallic interline fault at the line near end (0%) is taken as an example, and the per unit value is used for analysis. The following three case analyses are given to verify the performance of the mixed current limiting method proposed in this paper:

[0137] Case 1: the mixed current limiting method proposed in the application is adopted;

[0138] Case 2: the virtual inductance is introduced in the outer ring as the control current limiting, and the inductance is connected in series at 3 ms as the device current limiting;

[0139] Case 3: the adaptive current change rate is used to modify the voltage outer ring setting as the control current limiting, and the inductance is connected in series at 3 ms as the device current limiting.

[0140] Figure 8 A comparison chart of DC side fault current and MMC bridge arm current is given.

[0141] In case 2 and case 3, the bridge arm current exceeds the threshold value, causing the IGBT to be locked due to overcurrent, and the unlocking time is long, and the MMC loses controllability. In scheme 2, the virtual inductance is introduced, and due to the long control response, the current peak value is the largest, and based on the reference value, the method can reduce the DC fault current peak value and the bridge arm current peak value by 55.60% and 30.71% respectively. Taking case 3 as the reference value, the fault current peak value of the method is raised by 28.07% to 3.33p.u., and the bridge arm current is reduced by 19.56% to 1.85p.u., and the MMC still maintains controllability after the fault. The main reason is to reduce the boosting effect of AC feeding, k max 0.4 is selected, which weakens the current limiting effect of the DC side. The technical scheme provided by the above embodiment of the application can reduce the safety hazards caused by the overcurrent risk after the DC fault, and improve the safety of the system.

[0142] In summary, compared with the prior art, the application has the following beneficial effects:

[0143] 1. The hybrid current limiting control method for half-bridge type MMC DC side fault provided by the application realizes hybrid current limiting control by first injecting control current limiting and then injecting device current limiting, suppresses the rapidly growing fault current, starts the device current limiting when the control current limiting effect is the largest, increases the loop impedance, further suppresses the DC side fault current, and adaptively dynamically raises the DC side voltage to reduce the AC side current amplitude; during the control current limiting effect, the bridge arm current is raised, and after the device current limiting effect, the bridge arm current is effectively suppressed, ensuring that the IGBT current is in the device safety interval and reducing the safety hazards caused by DC and bridge arm overcurrent to related devices.

[0144] 2. In the method of the application, the sub-module bypass ratio k is rapidly adjusted to 1-m AC during the preliminary current limiting stage of the control current limiting, and then the preliminary suppression of the bridge arm current is realized, the k value is dynamically adjusted based on the linear current limiting suppression curve in real time during the dynamic current limiting stage of the control current limiting, and the device current limiting cooperates, which can quickly respond to the rapid increase of the current in the initial stage of the fault, and can accurately adapt the current limiting strength according to the current change, compared with pure control current limiting or device current limiting, the DC side fault current and the bridge arm current peak value can be significantly reduced, and the risk of device overcurrent damage is reduced.

[0145] 3. In the method of the application, when the control current limiting reaches the upper limit, that is, k maxThe device current limiting is started, and the control current limiting will be reset after the device current limiting is started, that is, the value of the sub-module bypass ratio k is determined by k max The AC side current is gradually reduced, and the quick response characteristic of the control current limiting and the influence of the AC side current on the bridge arm current are inhibited by the device current limiting, so that the problem of insufficient current limiting effect caused by the lack of cooperation in the prior art is solved.

[0146] 4, The starting control logic of the control current limiting strategy and the device current limiting strategy of the method is clear, and the monitoring and adaptive control execution logic is simple, which well balances the controllability and the strategy usability, and is beneficial to the engineering application promotion.

[0147] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, but not to limit the technical solutions, and those of ordinary skill in the art should understand that those who modify or equivalently replace the technical solutions of the present application without departing from the purpose and scope of the technical solutions should be covered in the scope of the claims of the present application.

Claims

1. A hybrid current limiting control method for a half-bridge MMC DC side fault, characterized in that, The converter valve of the MMC includes a, b, and c three phases, each phase including an upper bridge arm and a lower bridge arm; each bridge arm includes a plurality of sub-modules in series, each sub-module being a half-bridge sub-module; a plurality of damping modules for implementing current limiting and having controllable input states are respectively connected in series in each bridge arm of each phase, and the initial state of each damping module is in a bypass input state; The method comprises the following steps: S1: Real-time acquisition and monitoring of the bridge arm current of the MMC, preliminary judgment of whether the DC line has a fault; when it is judged that the DC line has a fault, step S2 is executed; S2: Starting of the control current limiting strategy, tracking of the size of the bridge arm current, dynamic control of the number of sub-modules input in the single-phase bridge arm, suppression of the fault current in the DC line, and analysis and determination of the upper limit of the control current limiting strategy; when the control current limiting strategy reaches the upper limit of current limiting, step S3 is executed; S3: Starting of the equipment current limiting strategy, tracking of the size of the bridge arm current amplitude, adaptive control of the number of damping modules input in the bridge arm, and further current limiting control of the fault current in the DC line.

2. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 1, characterized in that, In step S1, the bridge arm current change rate is used to preliminarily judge whether the DC line has a fault; the condition for judging that the DC line has a fault is that the bridge arm current change rate is greater than a preset threshold value. Wherein, |d ij / dt| is the absolute value of the bridge arm current rate of change; |d ij / dt| max is the peak value of the bridge arm current rate of change in the steady state; k1 is a preset reliability coefficient.

3. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 1, characterized in that, In step S2, after the control current limiting strategy is started, the number of sub-modules input in the single-phase bridge arm is dynamically controlled according to the set sub-module bypass proportion coefficient k: Wherein, n is the number of sub-modules input in the single-phase of the MMC during the DC line fault, which is a variable of dynamic control; N is the number of sub-modules input to maintain the stability of the DC side voltage under normal conditions, which is a constant value.

4. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 3, characterized in that, In step S2, the specific way of dynamically controlling the number of sub-modules input in the single-phase bridge arm by tracking the size of the bridge arm current is as follows: At the start of the control of the current limiting strategy, the initial value of the bypass proportional coefficient k of the sub-module is set as k = 1-m AC , m AC is the AC modulation ratio of the half-bridge MMC; then, by tracking the size of the bridge arm current, the value of the bypass proportional coefficient k of the sub-module is determined by using the current limiting suppression curve as follows: k = a • i ij + b, a = 2.5 (m AC + k max - 1), b = 3.75 - 3.75m AC - 2.75k max ; Wherein, a, b are current limiting suppression curve parameters; i ij Is the size of the bridge arm current; k max Is the upper limit value of the sub-module bypass proportional coefficient k; In the process of controlling the current-limiting strategy, when the value of the submodule bypass proportion coefficient k is determined, the number of single-phase input submodules n is determined according to the formula When the value of the submodule bypass proportion coefficient k reaches the upper limit value k max , the control of the current-limiting strategy reaches the upper limit of current limiting.

5. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 4, characterized in that, an upper limit value k of the sub-module bypass proportionality coefficient k max is determined as follows: When the half-bridge MMC is connected with the AC power grid on the AC side, the upper limit value k of the sub-module bypass proportion coefficient k is 0.4-0.8 max . When the AC side of the half-bridge MMC is connected to an AC weak grid, the upper limit value k of the sub-module bypass proportion coefficient k is 1-m max ~ 0.

4. AC ~ 0.

4.

6. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 1, characterized in that, For any lth damping module, including damping resistor R ADl , and switch device T ADl parallel with the damping resistor R ADl ; in the initial state, the switch device T ADl is in the on state, so that the damping resistor R ADl is bypassed, and the lth damping module is in the bypassed state; when the switch device T ADl is controlled to be off, the damping resistor R ADl is connected in the bridge arm, and the lth damping module enters the state of being put into operation.

7. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 6, characterized in that, The number of damping modules connected in series in each bridge arm of each phase is d, the damping resistance values of the damping modules are equal, and the sum R of the damping resistances of the d damping modules AD satisfies: L eq , C eq L and C represent the equivalent inductance and equivalent capacitance of the equivalent second-order RLC series circuit of the faulted loop, respectively.

8. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 7, characterized in that, In step S3, the specific way of adaptively controlling the number of damping modules input in the bridge arm by tracking the size of the bridge arm current amplitude is as follows: Set d interval factors r l , l = 1, 2, …, d, and 1.5 = r1 < r2 < … < rd d <2, according to the bridge arm current rated amplitude i armmax , the interval [1.5i armmax , 2i armmax ] is divided into d bridge arm current amplitude subintervals, respectively: [r1i armmax ,r2i armmax ), [r2i armmax ,r3i armmax ), …… [r d i armmax ,2i armmax ] Then, by tracking the magnitude of the bridge arm current amplitude i arm , if the value of the bridge arm current amplitude i arm falls into the bridge arm current amplitude sub-interval with r l as the lower limit, the number of damping modules put into the bridge arm is l.

9. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 8, characterized in that, The number of damping modules connected in series in each bridge arm of each phase is 4, and the values of the set 4 interval factors are as follows: r1＝1.5，r2＝1.6，r3＝1.7，r4＝1.8。 10. The hybrid current limiting control method for the half-bridge MMC DC side fault according to claim 3, characterized in that, After the device current-limiting strategy is started, the size of the bridge arm current is continuously tracked in the control current-limiting strategy, and with the decrease of the bridge arm current, the value of the submodule bypass proportionality coefficient k is gradually reduced from the upper limit value k max and thus the number of submodules put into the single-phase bridge arm is dynamically controlled.

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