High-voltage DC line pilot protection method and system based on virtual grounding resistance

Abstract

The present application relates to a method and system for longitudinal protection of high-voltage DC lines based on virtual grounding resistance, which belongs to the field of relay protection technology, and solves the problems of poor transition resistance resistance and easy misoperation in out-of-zone faults in existing methods. A method for longitudinal protection of high-voltage DC lines based on virtual grounding resistance includes: in an AC / DC hybrid system, obtaining the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter within a switching cycle after a fault occurs; determining the characteristic frequency of the circuit according to the conduction state information and the characteristic parameters; determining the virtual grounding resistance according to the characteristic frequency and the characteristic parameters; and determining the fault area according to the virtual grounding resistance. The technical solution provided by the present application can improve the reliability and sensitivity of fault judgment results.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection, and in particular to a high-voltage direct current line pilot protection method and system based on virtual grounding resistance. Background Art

[0002] At present, DC line protection is mainly divided into traveling wave protection, differential undervoltage protection and longitudinal current differential protection.

[0003] The above protection methods only consider the changes of various parameters in the DC line when a fault occurs. However, when a fault occurs and the converter is in the on state, the DC line will inevitably be affected by the AC power.

[0004] Since the effect of alternating current is not taken into account, the existing methods for line protection in the above scenarios are prone to problems such as poor transition resistance capability in the event of an in-zone fault, false operation in the event of an out-of-zone fault, and susceptibility to changes in the converter operating conditions and AC system operating mode. Summary of the invention

[0005] In view of the above analysis, the present application aims to propose a method and system for high-voltage DC line longitudinal protection based on virtual grounding resistance to solve at least one of the above technical problems.

[0006] The purpose of this application is mainly achieved through the following technical solutions:

[0007] On the one hand, the present application provides a high-voltage DC line pilot protection method based on virtual grounding resistance, comprising:

[0008] The AC / DC hybrid system obtains the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter within a commutation cycle after a fault occurs;

[0009] Determining a characteristic frequency of the circuit according to the conduction state information and the characteristic parameter;

[0010] Determining a virtual ground resistance according to the characteristic frequency and the characteristic parameter;

[0011] A fault area is determined according to the virtual grounding resistance.

[0012] Furthermore, the converter is a 12-pulse converter, the converter corresponds to four conduction states, and each conduction state corresponds to one conduction state information;

[0013] The characteristic parameters include: converter characteristic parameters and circuit characteristic parameters;

[0014] The converter characteristic parameters include: one or more of the three-phase voltage of the converter bus, the voltage of the preset node, the current of the converter transformer winding and the transformation ratio of the converter transformer;

[0015] The circuit characteristic parameters include: one or more of structural parameters, current, potential, inductance, equivalent inductance and equivalent resistance of each device in the circuit on the corresponding side.

[0016] Further, determining the characteristic frequency of the circuit according to the conduction state information and the characteristic parameter includes:

[0017] determining the impedance of the converter in the on-state according to the circuit characteristic parameters and the converter characteristic parameters corresponding to the on-state;

[0018] determining a characteristic frequency band of the circuit according to the impedance of the converter in the on state and the circuit characteristic parameters;

[0019] The characteristic frequency of the circuit is determined according to the characteristic frequency band of the circuit.

[0020] Further, determining the characteristic frequency band of the circuit according to the impedance of the converter in the on-state and the circuit characteristic parameter includes:

[0021] According to the impedance of the converter in the on state and the circuit characteristic parameters, an impedance characteristic equation is determined:

[0022]

[0023] Z rec is the DC line impedance, Z s is the impedance of the AC system, Z ac , Z dc are the equivalent impedance to ground of AC filter and DC filter respectively, Z d is the equivalent impedance of the smoothing reactor, Z c is the equivalent impedance of the converter;

[0024] According to the impedance characteristic equation, construct an inductive characteristic frequency band and / or a rectifier-side capacitive characteristic frequency band;

[0025] The perceptual characteristic frequency band is specifically:

[0026]

[0027] The capacitive characteristic frequency band is:

[0028]

[0029] Among them, ω 1.1 ,ω1.2 ,ω 1.3 ,ω 1.4 ,ω 1.5 and 1.6 for a 13 ω 13 +a 11 ω 11 +…+a1ω=0 solution;ω 2.2 ,ω 2.2 ,ω 2.3 ,ω 2.4 ,ω 2.5 and 2.6 for a 12 ω 12 +a 10 ω 10 +…+a0=0 solution.

[0030] Further, the characteristic frequency of the circuit is the minimum angular frequency and the second minimum angular frequency of the inductive characteristic frequency band on the rectifier side;

[0031] or,

[0032] The characteristic frequency of the circuit is the minimum angular frequency and the second minimum angular frequency of the rectifier-side capacitive characteristic frequency band.

[0033] Further, determining the virtual ground resistance according to the characteristic frequency and the characteristic parameter includes:

[0034] According to the characteristic parameters, construct a virtual ground resistance equation;

[0035] The virtual ground resistance equation is solved based on the characteristic frequency to obtain the virtual ground resistance.

[0036] Further, determining the fault area according to the virtual grounding resistance includes:

[0037] Determining whether the virtual grounding resistance is greater than a preset setting value;

[0038] When the virtual grounding resistance is less than a preset setting value, it is determined that a fault occurs in the DC line area;

[0039] Otherwise, it is determined that a fault occurred outside the DC line area.

[0040] In a second aspect, an embodiment of the present application provides a high-voltage DC line pilot protection system based on virtual grounding resistance, including: an acquisition module, a characteristic frequency calculation module and a data processing module;

[0041] The acquisition module is used to acquire the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter after a fault occurs in the AC / DC hybrid system within a commutation cycle;

[0042] The characteristic frequency calculation module is used to determine the characteristic frequency of the circuit according to the conduction state information and the characteristic parameter;

[0043] The data processing module is used to determine the virtual grounding resistance according to the characteristic frequency and the characteristic parameter; and determine the fault area according to the virtual grounding resistance.

[0044] Furthermore, the converter is a 12-pulse converter, the converter corresponds to four conduction states, and each conduction state corresponds to one conduction state information;

[0045] The characteristic parameters include: converter characteristic parameters and circuit characteristic parameters;

[0046] The converter characteristic parameters include: one or more of the three-phase voltage of the converter bus, the voltage of the preset node, the current of the converter transformer winding and the transformation ratio of the converter transformer;

[0047] The circuit characteristic parameters include: one or more of structural parameters, current, potential, inductance, equivalent inductance and equivalent resistance of each device in the circuit on the corresponding side.

[0048] Furthermore, the characteristic frequency calculation module is used to determine the impedance of the converter in the on-state based on the circuit characteristic parameters and the converter characteristic parameters corresponding to the on-state; determine the characteristic frequency band of the circuit based on the impedance of the converter in the on-state and the circuit characteristic parameters; and determine the characteristic frequency of the circuit based on the characteristic frequency band of the circuit.

[0049] Compared with the prior art, the technical effects of this application are:

[0050] 1. Existing DC transmission line protection technologies mostly perform fault identification based on the sudden change law of fault electrical quantities on the DC side, and their protection settings rely on experience, but lack fault modeling and analysis of the DC line and its sending and receiving end converters and AC systems. The present application determines the characteristic frequency based on the conduction state of the converter. The characteristic frequency is used to characterize the impact of the AC line on the DC line, and the impact of the AC line on the DC line is quantified. Based on the characteristic frequency, the virtual resistance is then determined. That is, in a short period of time, the DC fault line and its sending and receiving end converters and AC system will form a certain circuit topology, thereby realizing the modeling of DC line faults based on circuit principles. In summary, the present application is modeled based on real circuit conditions, so the judgment result has high reliability and sensitivity.

[0051] 2. Since the modeling of the prior art relies on experience, it is only applicable to the case where the grounding resistance is 0 or close to 0. Once the grounding resistance exists and is relatively large, the fault judgment sensitivity and accuracy will fluctuate greatly. The present application determines the characteristic frequency of the circuit based on the conduction state of the converter. Based on the measured impedance at the above characteristic frequency, the virtual grounding resistance equation is solved to calculate the grounding resistance during a fault in the DC line area using single-ended quantity information of different frequencies, and construct a protection criterion based on the difference in the calculated value of the grounding resistance during a short circuit inside and outside the area. In summary, the present application performs fault judgment based on the grounding resistance, so the judgment result has high reliability and sensitivity.

[0052] Other features and advantages of the present application will be described in the following description, and some will become apparent from the description, or will be understood by practicing the present application. The purpose and other advantages of the present application can be realized and obtained by the structures specifically pointed out in the written description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The accompanying drawings are only for the purpose of illustrating specific embodiments and are not to be considered as limiting the present application. The same reference symbols denote the same components throughout the accompanying drawings.

[0054] Figure 1 A flow chart of a high-voltage DC line pilot protection method based on virtual grounding resistance provided in an embodiment of the present application;

[0055] Figure 2 A schematic diagram of an AC / DC hybrid system provided in an embodiment of the present application;

[0056] Figure 3 The principle wiring diagram of the 12-pulse inverter on the rectifier side provided in the embodiment of the present application;

[0057] Figure 4 A specific structural diagram of the AC and DC filters and DC smoothing reactor provided in the embodiments of the present application;

[0058] Figure 5 A fault component network when a fault occurs within a zone provided by an embodiment of the present application;

[0059] Figure 6 A fault component network for an out-of-zone fault provided in an embodiment of the present application;

[0060] Figure 7 A structural schematic diagram of a specific example of a high-voltage DC line longitudinal connection based on a virtual grounding resistor provided in an embodiment of the present application;

[0061] Figure 8 is R when a fault occurs through different transition resistances in a specific example gM ;

[0062] Fig. 9 is R when a fault occurs through different transition resistances in a specific example gN ;

[0063] Fig.10 is R when a fault occurs at different locations in a specific example gM ;

[0064] Fig.11 is R when a fault occurs at different locations in a specific example gN ;

[0065] Fig.12 is R when a fault occurs through different transition resistances in a specific example gM ;

[0066] Fig.13 is R when a fault occurs through different transition resistances in a specific example gN . DETAILED DESCRIPTION

[0067] The preferred embodiments of the present application are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present application and are used together with the embodiments of the present application to illustrate the principles of the present application, and are not used to limit the scope of the present application.

[0068] At present, DC line protection is mainly divided into traveling wave protection, differential undervoltage protection and longitudinal current differential protection. The traveling wave protection is based on the fault traveling wave voltage and current in the initial stage of the fault. Since the fault voltage and current change significantly in the initial stage of the traveling wave and are not affected by the control of the DC system, in actual engineering, the single-ended protection method based on the initial traveling wave of the fault is generally used as the main protection of the DC line. However, the traveling wave waveform is complex and the data processing process is relatively time-consuming, which will cause a delay in the protection action. At the same time, the traveling wave protection does not consider the impact of the AC system on the DC line, which reduces the accuracy of determining the fault location. In addition, the traveling wave protection needs to consider the impact of the proximal metallic fault outside the zone, resulting in a higher setting value. When a high-resistance fault occurs in the zone, the traveling wave protection cannot correctly and reliably identify the fault.

[0069] Differential undervoltage protection uses the voltage change rate and voltage reduction amplitude after the fault occurs to form the protection criterion. Compared with traveling wave protection, differential undervoltage protection has higher selectivity and sensitivity. Differential undervoltage protection does not take into account the impact of the AC system on the DC line, which reduces the accuracy of determining the fault location. The voltage change rate has a delay, which leads to a delay in the protection action. At the same time, the fault voltage and current are difficult to solve by analytical methods, resulting in the need to determine the protection setting by exhaustive simulation methods, thereby increasing the amount of data processing. Even if the solution is obtained, since the data is formed after the fault occurs, erroneous protection actions will occur. In addition, the ability of differential undervoltage protection to withstand transition resistance is still very limited.

[0070] The longitudinal current differential protection is a protection scheme based on the sum of the currents at both ends of the DC line, which is used to remove high-resistance faults that the traveling wave protection and differential undervoltage protection fail to operate. The longitudinal current differential protection does not take into account the impact of the AC system on the DC line, which reduces the accuracy of determining the fault location. In actual operation, the presence of distributed capacitance in the DC line will cause the protection to malfunction under out-of-zone faults or interference. In addition, the longitudinal current differential protection requires the exchange of electrical quantity information at both ends, and has high requirements for communication channels and data synchronization.

[0071] Based on the above problems and scenarios, the present application provides a high-voltage DC line longitudinal protection method based on virtual grounding resistance, such as Figure 1 As shown, the following steps are included:

[0072] Step 1: The AC / DC hybrid system obtains the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter within one commutation cycle after a fault occurs.

[0073] In the embodiment of the present application, the converter is a 12-pulse converter, and the converter corresponds to four conduction states, and each conduction state corresponds to a conduction state information; the characteristic parameters include: converter characteristic parameters and circuit characteristic parameters; the converter characteristic parameters include: one or more of the three-phase voltage of the converter bus, the voltage of the preset node, the current of the converter transformer winding and the transformation ratio of the converter transformer; the circuit characteristic parameters include: one or more of the structural parameters, current, inductance, equivalent inductance and equivalent resistance of each device in the circuit on one side. In actual use, any one of the four converters is in the conduction state, and the method in the embodiment of the present application can be implemented.

[0074] The circuit structure of the embodiment of the present application is as follows Figure 2As shown, MN is a DC transmission line, the circuit on the left side of the M node is the rectifier side, and the circuit on the right side of the N node is the inverter side. The rectifier side and the inverter side are respectively provided with two converters, an AC system, an AC filter, a smoothing reactor, and a DC filter. The circuit structures on both sides are exactly the same, and the method of the present application can be implemented on either side. For ease of explanation, the present application embodiment takes the rectifier side as an example for explanation. Among them, the specific structure of the converter is as follows Figure 3 As shown, it includes: a D-bridge converter and a Y-bridge converter.

[0075] Among them, the D-bridge converter includes converter valves VTD1-VTD6; wherein, three series branches are formed by VTD1 and VTD4, VTD3 and VTD6, and VTD5 and VTD2, the cathodes of VTD1, VTD3 and VTD5 are connected, and the anodes of VTD4, VTD6 and VTD2 are connected; the Y-bridge converter includes converter valves VTY1-VTY6; wherein, three series branches are formed by VTY1 and VTY4, VTY3 and VTY6, and VTY5 and VTY2, the cathodes of VTY1, VTY3 and VTY5 are connected, and the anodes of VTY4, VTY6 and VTY2 are connected.

[0076] In the 12-pulse converter on the rectifier side, the conduction state of each valve arm can be divided into the following four types:

[0077] 1) On state 1: In the D-bridge converter, two converter valves with adjacent numbers are on; in the Y-bridge converter, two converter valves with the same number as the converter valves that are on in the D-bridge converter are on; specifically: VTD1 is on VTD2, and VTY1 is on VTY2; or, VTD2 is on VTD3, and VTY2 is on VTY3; or, VTD3 is on VTD4, and VTY3 is on VTY4; or, VTD4 is on VTD5, and VTY4 is on VTY5; or, VTD5 is on VTD6, and VTY5 is on VTY6; or, VTD6 is on VTD1, and VTY6 is on VTY1; at this time, the converter impedance is Z cop1 .

[0078] 2) On state 2: In the D-bridge converter, two adjacently numbered converter valves are on; in the Y-bridge converter, three adjacently numbered converter valves are on; and the numbers of the converter valves on in the D-bridge converter correspond to the same numbers of the first two converter valves on in the Y-bridge converter; specifically: VTD1 is on with VTD2, and VTY1, VTY2, and VTY3 are on; or, VTD2 is on with VTD3, and VTY2, VTY3, and VTY4 are on; or, VTD3 is on with VTD4, and VTY3, VTY4, and VTY5 are on; or, VTD4 is on with VTD5, and VTY4, VTY5, and VTY6 are on; or, VTD5 is on with VTD6, and VTY5, VTY6, and VTY1 are on; or, VTD6 is on with VTD1, and VTY6, VTY1, and VTY2 are on; at this time, the converter impedance is Z cop2 .

[0079] 3) On state 3: In the D-bridge converter, two adjacent converter valves are on; in the Y-bridge converter, two adjacent converter valves are on; and the number of the latter converter valve on in the D-bridge converter is the same as the number of the former converter valve on in the Y-bridge converter; specifically, VTD1 is on with VTD2, and VTY2 is on with VTY3; or, VTD2 is on with VTD3, and VTY3 is on with VTY4; or, VTD3 is on with VTD4, and VTY4 is on with VTY5; or, VTD4 is on with VTD5, and VTY5 is on with VTY6; or, VTD5 is on with VTD6, and VTY6 is on with VTY1; or, VTD6 is on with VTD1, and VTY1 is on with VTY2; at this time, the converter impedance is Z cop3 .

[0080] 4) On state 4: In the D-bridge converter, three adjacently numbered converter valves are on; in the Y-bridge converter, two adjacently numbered converter valves are on; and the numbers of the converter valves on in the Y-bridge converter correspond to the same numbers of the last two converter valves on in the D-bridge converter; specifically, VTD1, VTD2 and VTD3 are on, and VTY2 and VTY3 are on; or, VTD2, VTD3 and VTD4 are on, and VTY3 and VTY4 are on; or, VTD3, VTD4 and VTD5 are on, and VTY4 and VTY5 are on; or, VTD4, VTD5 and VTD6 are on, and VTY5 and VTY6 are on; or, VTD5, VTD6 and VTD1 are on, and VTY6 and VTY1 are on; or, VTD6, VTD1 and VTD2 are on, and VTY1 and VTY2 are on; at this time, the converter impedance is Z cop4 .

[0081] In the above conduction state, other converter valves not described are all in the off state. The information representing the above conduction state is the conduction state information of the converter.

[0082] Rectifier side AC system, AC filter, smoothing reactor and DC filter such as Figure 4 As shown. Among them, C1, C2, L1 and L2 are the structural parameters of the DC filter; C3, C4, C5, C6, R3, R4, R5, L3 and L4 are the structural parameters of the AC filter; R W1 With L W1 They are the equivalent resistance and equivalent inductance of the rectifier side AC system. The structural parameters are set when each component leaves the factory and can be regarded as fixed settings of each component. Among them, C represents capacitance, R represents resistance, and L represents inductance.

[0083] Step 2: Determine the characteristic frequency of the circuit based on the conduction state information and characteristic parameters.

[0084] In the embodiment of the present application, step 2 and the specific process are:

[0085] According to the converter characteristic parameters corresponding to the on-state of the circuit characteristic parameters, the impedance of the converter in the on-state is determined; according to the impedance of the converter in the on-state and the circuit characteristic parameters, the characteristic frequency band of the circuit is determined; according to the characteristic frequency band of the circuit, the characteristic frequency of the circuit is determined.

[0086] Specifically, taking the rectifier side conduction state 2 as an example, when the 12-pulse inverter is in the conduction state 2, combined with Figure 3 We can get equations (1)-(8):

[0087] i ad =i bd (1)

[0088] i cd +i d =i bd (2)

[0089] i ay +i by =i d (3)

[0090] i cy =-i d (4)

[0091]

[0092]

[0093]

[0094]

[0095] In the formula, u a 、u b 、u c is the three-phase voltage of the commutation bus on the rectifier side; u d 、u d1 、u d2 are the voltages at positions d1, d2 and d3 in Figure 3 respectively; i ad 、i bd 、i cd 、i ay 、i by 、i cy They are Figure 3 Medium voltage source k d u a , k d u b , k d u c , k y u a , k y u b , the current of the converter transformer winding where kyuc is located, and its positive direction is the associated reference direction of the above voltage source; y k is the transformation ratio of the Y / Y connection converter transformer connected to the Y bridge converter; d is the transformation ratio of the Y / △ connection converter transformer connected to the D bridge converter; L r L is the inductance of the converter transformer converted to the valve side; d is the inductance of the smoothing reactor; i d is the current flowing through the smoothing reactor.

[0096] according to Figure 2 It can be seen that the three-phase voltage of the commutation bus and the three-phase current of the AC line LK exist in the following equations:

[0097]

[0098]

[0099] Where r1 and l1 are the positive sequence resistance and positive sequence inductance of the AC line LK respectively; sa 、e sb and e sc is the equivalent three-phase potential of the AC system S1; C a , C b and C c is the three-phase current of the rectifier side AC filter.

[0100] Based on equations (1)-(10), the impedance characteristic equation of the on-state 2 is determined as:

[0101]

[0102] Where ω is the angular frequency.

[0103] The converter impedance characteristic equation in other conduction states is:

[0104]

[0105] Wherein, Zop1, Zop3 and Zop4 are impedances when the converter is in conduction state 1, conduction state 3 and conduction state 4, respectively. In the above manner, the influence of the AC system on the virtual resistance can be added to the impedance characteristic equation.

[0106] according to Figure 4 It can be seen that the impedance characteristic equations of the DC filter, smoothing reactor, rectifier-side AC system and AC filter are:

[0107]

[0108] Z d =jωL d (12)

[0109] Z s1 =R W1 +jωL W1 (13)

[0110]

[0111] The symbol “ / / ” indicates the sum of parallel resistances.

[0112] When a fault occurs in the DC line, the equivalent impedance Z of the DC filter expressed by equations (10)-(13) is dc , DC smoothing reactor equivalent impedance Z d , the equivalent impedance Z of the AC system at the sending end s1 And the equivalent impedance Z of the AC filter ac Substitute the following equivalent impedance Z of the back system at the protection installation location: rec The expression of the denominator is rationalized and its imaginary part is taken, and the numerator and denominator are arranged as a polynomial about the frequency ω, where a 13 , a 12 , a 11 , a 10 , a9, a8, a7, a6, a5, a4, a3, a2, a1, a0 are the coefficients before the 13th to 0th order terms of ω, and their expressions are related to the model parameters of equations (10)-(13), and the impedance characteristic equation of the rectifier side is obtained as follows:

[0113]

[0114] According to the impedance characteristic equation of the rectifier side, the impedance of the rectifier side is obtained in the inductive band and the capacitive band respectively:

[0115] Sensitive frequency band range:

[0116]

[0117] Capacitive frequency band range:

[0118]

[0119] In the formula, ω 1.1 ,ω 1.2 ,ω 1.3 ,ω 1.4 ,ω 1.5 and 1.6 for a 13 ω 13 +a 11 ω 11 +…+a1ω=0 solution;ω 2.2 ,ω 2.2 ,ω 2.3 ,ω 2.4 ,ω 2.5 and 2.6 for a 12 ω 12 +a 10 ω 10 +…+a0=0 solution.

[0120] It is known that the characteristic harmonic angular frequency of the DC voltage during normal operation is 1200kπrad / s (k=1,2,……), so the characteristic frequency band range of the inductive band is:

[0121] ω hvdcL =ω L ∩(ω=1200kπ)

[0122] The characteristic frequency band range in the capacitive band is:

[0123] ω hvdcC =ω C ∩(ω=1200kπ)

[0124] In actual scenarios, both the characteristic frequency band of the inductive frequency band and the characteristic frequency band of the capacitive frequency band can be used to determine the characteristic frequency of the circuit. The inductive characteristic frequency of the circuit is the minimum angular frequency and the second minimum angular frequency of the inductive characteristic frequency band on the rectifier side; or the capacitive characteristic frequency of the circuit is the minimum angular frequency and the second minimum angular frequency of the capacitive characteristic frequency band on the rectifier side.

[0125] During normal operation, the right boundary of the characteristic harmonic interval of the DC line voltage in the inductive frequency band is positive infinity, while the characteristic harmonic interval in the capacitive frequency band is a finite interval, so the former has more transient component information available. Since the transient component of the DC line voltage under fault conditions is greater than that under normal operation, it should be prioritized to ensure that there is sufficient transient component information to drive the specific instance during normal operation of the DC line. Therefore, during the protection operation, the minimum and second minimum angular frequencies ω1 and ω2 on the inductive characteristic harmonic frequency band are used for the calculation of the protection action criterion.

[0126] Step 3: Determine the virtual ground resistance according to the characteristic frequency and characteristic parameters.

[0127] In the embodiment of the present application, step 3 includes: constructing a virtual ground resistance equation according to the characteristic parameters, solving the virtual ground resistance equation based on the impedance measurement value at the characteristic frequency, and obtaining the virtual ground resistance.

[0128] Specifically, based on the model relationship between the internal and external faults in the system area and the characteristic parameters, a virtual impedance equation group is constructed:

[0129]

[0130] Z eq1 =Z dc / / [Z d +Z c +(Z ac / / Z s2 )]

[0131] Among them, Z line is the DC line impedance, Z rec2 is the measured impedance of the DC line protection installation on this side, x is the percentage of the fault distance from the fault point to the protection installation on this side, R gM is the virtual ground resistance. s2 is the impedance of the inverter side AC system, Z ac , Z dc are the equivalent impedance to ground of AC filter and DC filter respectively, Z d is the equivalent impedance of the smoothing reactor, Z c is the equivalent impedance of the converter.

[0132] Substituting the characteristic frequencies ω1 and ω2 into the above equations respectively, we obtain:

[0133]

[0134]

[0135] Combining equation (15) and equation (16), we can get:

[0136]

[0137] According to formula (17), the only unknown quantity in the virtual grounding resistance equation is the fault position. The fault position can be obtained by performing nonlinear least squares estimation on the virtual grounding resistance equation, and the calculated fault position can be substituted into formula (15) or formula (16) to obtain the virtual grounding resistance R gM .

[0138] Step 4: Determine the fault area based on the virtual ground resistance.

[0139] In the embodiment of the present application, step 4 includes:

[0140] It is determined whether the virtual grounding resistance is greater than the preset setting value; when the virtual grounding resistance is less than the preset setting value, it is determined that a fault occurs in the DC line area; otherwise, it is determined that a fault occurs outside the DC line area.

[0141] The process of determining the set value is as follows:

[0142] When a fault occurs in the DC line area, the fault component network of the AC / DC hybrid system is as follows: Figure 5 As shown, Figure 5 The DC busbar on the rectifier side is the M end of the DC line, and the DC busbar on the inverter side is the N end of the DC line. line1 is the impedance from the fault point to the M terminal; Z line2 is the impedance from the fault point to the N terminal; Z s1 , Z s2 are the impedances of the AC systems on the rectifier and inverter sides, respectively, and Z ac , Z dc are the equivalent impedance to ground of AC filter and DC filter respectively, Z d is the equivalent impedance of the smoothing reactor; R M is the transition resistor.

[0143] according to Figure 5 It can be seen that the voltage and current at the M terminal have the following relationship:

[0144]

[0145] In the formula, △u m , △i m are the voltage and current at the M terminal respectively; x is the percentage of the fault distance from the fault point to the protection installation on this side (x∈[0,1]); Z rec2 The impedance measured at the DC line protection installation on this side; Z line is the DC line impedance. Among them, this side refers to the rectifier side.

[0146] When a fault occurs outside the rectifier side of the DC line, the fault component network of the AC / DC hybrid system is as follows: Figure 6 shown.

[0147] according to Figure 6 It can be seen that the voltage and current at the M terminal have the following relationship:

[0148]

[0149] In the formula, R M is a constant approaching +∞, △u m , △i m are the voltage and current at the M terminal respectively; Z line is the DC line impedance; x is the percentage of the fault distance from the fault point to the protection installation on this side (x∈[0,1]); Z rec2 The impedance measured at the DC line protection installation on this side; Z line is the DC line impedance; Z line1 Z is the impedance from the fault point to the protection installation on this side; line2 Z is the impedance from the fault point to the opposite protection installation; s2 is the impedance of the AC system on the inverter side. Among them, this side refers to the rectifier side, and the opposite side refers to the inverter side.

[0150] Since the virtual grounding resistance is equal to the transition resistance in value, it can be seen from the two situations of internal fault and external fault that when the DC line has an internal fault, the virtual grounding resistance is the actual transition resistance and is a finite value. When the DC line has an external fault, the virtual grounding resistance approaches infinity. Based on this, the protection action criterion for the DC line internal fault is constructed:

[0151] R gM <R set When , the fault occurs in the area, otherwise the fault occurs outside the area;

[0152] or,

[0153] R gN <R set When , the fault occurs within the area, otherwise the fault occurs outside the area.

[0154] R gN is the virtual ground resistance calculated based on the voltage and current data of the N terminal; R set is the set value.

[0155] The protection setting value is affected by the maximum transition resistance of the DC line fault area, and a certain reliability coefficient needs to be considered, thus:

[0156] R set =K rel R max

[0157] In the formula, Krel is the reliability coefficient, considering the influence of noise interference and other factors, K rel Take it as 1.5; R max It is the maximum transition resistance when a fault occurs in the DC line zone. The reliability coefficient and the maximum transition resistance ensure that the fault in the zone will not be identified as an out-of-zone fault, thus avoiding malfunction of the protection circuit.

[0158] like Figure 7 As shown, the embodiment of the present application provides a specific example of a high-voltage DC line longitudinal connection based on a virtual grounding resistance, including: an acquisition module 701, a characteristic frequency calculation module 702 and a data processing module 703;

[0159] The acquisition module 701 is used to obtain the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter after a fault occurs in the AC / DC hybrid system within a commutation cycle;

[0160] The characteristic frequency calculation module 702 is used to determine the characteristic frequency of the circuit according to the conduction state information and characteristic parameters;

[0161] The data processing module 703 is used to determine the virtual grounding resistance according to the measured impedance and characteristic parameters at the characteristic frequency; and determine the fault area according to the virtual grounding resistance.

[0162] In the embodiment of the present application, the converter is a 12-pulse converter, the converter corresponds to four conduction states, and each conduction state corresponds to a conduction state information;

[0163] The characteristic parameters include: converter characteristic parameters and circuit characteristic parameters;

[0164] The converter characteristic parameters include: one or more of the three-phase voltage of the converter bus, the voltage of the preset node, the current of the converter transformer winding and the transformation ratio of the converter transformer;

[0165] The circuit characteristic parameters include: one or more of the structural parameters, current, potential, inductance, equivalent inductance and equivalent resistance of each device in the circuit on one side.

[0166] In the embodiment of the present application, the characteristic frequency calculation module 702 is used to determine the impedance of the converter in the on-state according to the circuit characteristic parameters and the converter characteristic parameters corresponding to the on-state; determine the characteristic frequency band of the circuit according to the impedance of the converter in the on-state and the circuit characteristic parameters; and determine the characteristic frequency of the circuit according to the characteristic frequency band of the circuit.

[0167] In order to verify the correctness of the high-voltage DC line longitudinal protection method based on virtual grounding resistance provided by the above embodiment of the present invention, this embodiment provides a specific example: the main parameters of the AC / DC hybrid system are shown in Table 1, with the fault occurrence time as time zero.

[0168] Table 1 Main parameters of AC / DC hybrid system

[0169]

[0170] Substituting the parameters of each component in Table 1 into the impedance characteristic equation of the rectifier side, it can be calculated that the minimum angular frequency of the system is inductive, which is 1200πrad / s. Therefore, ω1 can be taken as 1200πrad / s and ω2 can be taken as 2400πrad / s. The moment when the fault occurs is taken as the zero moment.

[0171] The first setting scenario of this embodiment is: suppose that a fault occurs at 50% of the positive DC line through different transition resistances, and the set transition resistance range is 0-300Ω. gM , R gN Respectively Figure 2 , Figure 3 shown.

[0172] according to Figure 2 and Figure 3 It can be seen that the simulation results of time, actual transition resistance and virtual grounding resistance form a 45° inclined plane, which shows that the actual transition resistance at different time sections is consistent with the virtual grounding resistance, and as the actual transition resistance increases, the relative error first increases and then decreases. When the actual transition resistance is 40Ω, R gM At t = 9.9ms, the relative error reaches the maximum value of 2.85%; when the actual transition resistance is 200Ω, R gN At t=9.7ms, the relative error reaches the maximum value of 2.25%.

[0173] Depend on Figure 2 and Figure 3 It can be seen that under different fault conditions, as the transition resistance increases, R gM With R gN It keeps increasing. According to the judgment criteria of the embodiment of the present application, the protection recognizes that a fault occurs in the DC line area. Fig.10 It can be seen that when a fault occurs in the DC line and the transition resistance is 300Ω, R gM The maximum value is 298.44 when t = 0.5ms. When the transition resistance is 300Ω, Fig.11 As shown, R gN The maximum value is 304.21 at t = 0.2 ms. According to the above analysis, this method has high sensitivity and rapid identification ability when a high-resistance fault occurs in the DC line area.

[0174] The second scenario set in this embodiment is: suppose that a negative line fault occurs in the DC line at different positions away from the M terminal, and the transition resistance is 300Ω. gM , R gN Respectively Figure 4 , Figure 5 shown.

[0175] Depend on Figure 4 and Figure 5 It can be seen that when a negative line fault occurs at different locations from the M end, R gM With R gN The fluctuation degree is small. According to the judgment criteria of the embodiment of the present application, the protection identifies that a fault occurs in the DC line area. When a negative line fault occurs at a position 60% away from the M end, R gM The maximum value is 298.88 at t = 0.5ms. When a negative line fault occurs at 20% of the distance from the M end, R gN The maximum value is 343.08 at t = 0.1ms, but it is still far less than the threshold value. From the above analysis, it can be seen that the protection criterion is not affected by the fault location and still has a high sensitivity when a high-resistance fault occurs at the end of the line.

[0176] The third setting scenario of this embodiment is: Figure 2 A phase grounding fault occurs at f1, and the transition resistance changes in the range of 0 to 300Ω. At t = 0ms, the Y-bridge converter is in the VTY1 and VTY2 conduction state, and the D-bridge converter is in the VTD1 and VTD2 conduction state. Under the triggering signals of the Y-bridge converter VTY3 and the D-bridge converter VTD3, the Y-bridge converter and the D-bridge converter are commutated successively. When the voltage borne by the D-bridge converter VTD1 changes from negative to positive for the first time, the conduction states of the Y-bridge converter and the D-bridge converter are shown in Table 2.

[0177] Table 2 Converter conduction state when phase A ground fault occurs at f1

[0178]

[0179]

[0180] It can be seen from Table 2 that when the transition resistance is 0Ω, the Y-bridge converter VTY1 fails to restore its blocking capability during the reverse voltage period, and the Y-bridge converter fails to commutate. When the transition resistance changes in the range of 50 to 300Ω, the conduction state of the Y-bridge converter and the D-bridge converter is the same as that during normal operation, and it can be determined that both the Y-bridge converter and the D-bridge converter are commutating normally. gM , R gN Respectively Fig.12 , Fig.13 shown.

[0181] Depend on Figure 6 and Figure 7 It can be seen that under this fault condition, as the transition resistance increases, the virtual grounding resistance at different time sections first increases and then decreases, and both are greater than the action threshold.

[0182] according to Figure 6 It can be seen that when the transition resistance is 40Ω, R gM The minimum value is 4782.18 when t=0.4ms. Figure 7 As shown, R gN The minimum value is 2643.99 when t=3ms and the transition resistance is 300Ω, which is much larger than the action threshold, indicating that no fault occurs in the MN area of ​​the DC line and the protection is reliable and does not operate.

[0183] Considering the case where there is a synchronization error between the inverter side data and the rectifier side data, combined with formula 17, it can be seen that R gN If the value is always greater than the threshold value, it can be determined that a fault has occurred in the reverse direction of the DC line. Since the current shutdown and system restart after the DC line fault are completed on the rectifier side, it is only necessary to transmit the judgment result of the fault direction from the inverter side to the rectifier side to identify it as an out-of-zone fault. According to the above analysis, the proposed scheme does not need to transmit electrical quantity information and is not affected by commutation failure and data synchronization error.

[0184] The above is only a preferred specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed in the present application should be covered within the protection scope of the present application.

Claims

1. A method for pilot protection of a high voltage DC line based on virtual grounding resistance, characterized in that: include: The AC / DC hybrid system obtains the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter within a commutation cycle after a fault occurs; The inverter is a 12-pulse inverter, the inverter corresponds to four conduction states, and each conduction state corresponds to one conduction state information; The characteristic parameters include: converter characteristic parameters and circuit characteristic parameters; the converter characteristic parameters include: one or more of the three-phase voltage of the converter bus, the voltage of the preset node, the current of the converter transformer winding and the transformation ratio of the converter transformer; the circuit characteristic parameters include: one or more of the structural parameters, current, potential, inductance, equivalent inductance and equivalent resistance of each device in the circuit on the corresponding side; Determining the characteristic frequency of the circuit according to the conduction state information and the characteristic parameters includes: determining the impedance of the converter in the conduction state according to the circuit characteristic parameters and the converter characteristic parameters corresponding to the conduction state; determining the characteristic frequency band of the circuit according to the impedance of the converter in the conduction state and the circuit characteristic parameters, including: determining the impedance characteristic equation according to the impedance of the converter in the conduction state and the circuit characteristic parameters: Z rec is the DC line measurement impedance, Z s is the impedance of the AC system, Z ac , Z dc are the equivalent impedance to ground of AC filter and DC filter respectively, Z d is the equivalent impedance of the smoothing reactor, Z c is the equivalent impedance of the converter; according to the impedance characteristic equation, an inductive characteristic frequency band and / or a rectifier-side capacitive characteristic frequency band is constructed; the inductive characteristic frequency band is specifically: The capacitive characteristic frequency band is: Among them, ω 1.1 ,ω 1.2 ,ω 1.3 ,ω 1.4 ,ω 1.5 and 1.6 for a 13 ω 13 +a 11 ω 11 +…+a1ω=0 solution;ω 2.1 ,ω 2.2 ,ω 2.3 ,ω 2.4 ,ω 2.5 and 2.6 for a 12 ω 12 +a 10 ω 10 +…+a0=0 solution; determining the characteristic frequency of the circuit according to the characteristic frequency band of the circuit; Determining a virtual ground resistance according to the characteristic frequency and the characteristic parameter; A fault area is determined according to the virtual grounding resistance.

2. The method according to claim 1, characterized in that: The characteristic frequency of the circuit is the minimum angular frequency and the second minimum angular frequency of the inductive characteristic frequency band on the rectifier side; or, The characteristic frequency of the circuit is the minimum angular frequency and the second minimum angular frequency of the rectifier-side capacitive characteristic frequency band.

3. The method according to claim 1, characterized in that The step of determining the virtual ground resistance according to the characteristic frequency and the characteristic parameter includes: According to the characteristic parameters, construct a virtual ground resistance equation; The virtual ground resistance equation is solved based on the characteristic frequency to obtain the virtual ground resistance.

4. The method according to claim 1, characterized in that: The step of determining the fault area according to the virtual grounding resistance includes: Determining whether the virtual grounding resistance is greater than a preset setting value; When the virtual grounding resistance is less than a preset setting value, it is determined that a fault occurs in the DC line area; Otherwise, it is determined that a fault occurred outside the DC line area.

5. A high voltage DC line pilot protection system based on virtual grounding resistance, based on the method according to any one of claims 1 to 4, characterized in that: include: Acquisition module, characteristic frequency calculation module and data processing module; The acquisition module is used to acquire the conduction state information of the converter and the characteristic parameters of the circuit on one side of the converter after a fault occurs in the AC / DC hybrid system within a commutation cycle; The characteristic frequency calculation module is used to determine the characteristic frequency of the circuit according to the conduction state information and the characteristic parameter; The data processing module is used to determine the virtual grounding resistance according to the characteristic frequency and the characteristic parameter; and determine the fault area according to the virtual grounding resistance.

6. The system according to claim 5, characterized in that The inverter is a 12-pulse inverter, the inverter corresponds to four conduction states, and each conduction state corresponds to one conduction state information; The characteristic parameters include: converter characteristic parameters and circuit characteristic parameters; The converter characteristic parameters include: one or more of the three-phase voltage of the converter bus, the voltage of the preset node, the current of the converter transformer winding and the transformation ratio of the converter transformer; The circuit characteristic parameters include: one or more of structural parameters, current, potential, inductance, equivalent inductance and equivalent resistance of each device in the circuit on the corresponding side.

7. The system according to claim 6, characterized in that The characteristic frequency calculation module is used to determine the impedance of the converter in the on-state according to the circuit characteristic parameters and the converter characteristic parameters corresponding to the on-state; determine the characteristic frequency band of the circuit according to the impedance of the converter in the on-state and the circuit characteristic parameters; The characteristic frequency of the circuit is determined according to the characteristic frequency band of the circuit.

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