WO2020147572A1

WO2020147572A1 - Auxiliary circuit for hybrid direct-current circuit, and method and system for identifying property of fault of multi-port flexible direct current grid

Info

Publication number: WO2020147572A1
Application number: PCT/CN2019/130085
Authority: WO
Inventors: 邹贵彬; 张烁; 魏秀燕; 高厚磊
Original assignee: 山东大学
Priority date: 2018-12-24
Filing date: 2019-12-30
Publication date: 2020-07-23
Also published as: CN109713653A; CN109713653B

Abstract

An auxiliary circuit for a hybrid direct-current circuit, and a method and system for identifying the property of a fault of a multi-port flexible direct current grid. The auxiliary circuit is added to Nc IGBT sub-modules on a transfer branch of the hybrid direct current circuit breaker close to an overhead line side. The auxiliary circuit comprises: thyristors, a resistor Rg, and a fast mechanical switch. The thyristors are respectively connected in series between an overhead line and a negative electrode of a snubber capacitor of a first IGBT sub-module snubber circuit and between snubber capacitors of two adjacent IGBT sub-module snubber circuits. A positive electrode of a snubber capacitor of an Nc-th IGBT sub-module snubber circuit is sequentially connected in series to the resistor and the fast mechanical switch, and is then grounded wherein Nc is a set value. The auxiliary circuit of the hybrid direct current circuit breaker implements injection of a feature signal to identify transient faults and permanent faults, and can implement the fault localization of the permanent faults.

Description

Auxiliary circuit of hybrid DC circuit breaker, method and system for identifying fault nature of multi-terminal flexible DC power grid

Technical field

The invention relates to the technical field of line fault recovery of a flexible DC power grid, and in particular to an auxiliary circuit of a hybrid DC circuit breaker, a method and system for identifying fault properties of a multi-terminal flexible DC power grid.

Background technique

The statements in this section merely provide background information related to the present invention, and do not necessarily constitute prior art.

With the development of DC circuit breakers and the requirements of modern power grids for operational flexibility, the multi-terminal flexible DC grid based on overhead lines has outstanding advantages in large-scale clean energy generation and grid connection and large-capacity transmission. It is an important supplement to the AC grid and is currently under construction. The Zhangbei project is a four-terminal flexible DC grid that transmits power through overhead lines.

The inventor found that, compared with DC cables, overhead lines have a higher probability of failure due to exposure to the air. According to statistics, most of the faults in the power system occur on overhead lines. The current fault isolation methods of flexible DC systems are mainly based on AC circuit breakers, fault self-clearing converters and DC circuit breakers. The fault isolation scheme based on DC circuit breaker is considered to be the best solution to deal with multi-terminal DC grid DC line faults. Hybrid DC circuit breaker is a relatively mature topology for engineering applications. The DC circuit breaker used in Zhangbei project is 500kV Hybrid DC circuit breaker.

Since most of the overhead line faults are transient faults, the AC overhead line in the AC system generally needs to be equipped with reclosing technology. The method of reclosing the AC circuit breaker after a short delay after the fault can be used to clear the transient fault and shorten the power supply. The recovery time greatly improves the reliability of power supply. According to statistics, the probability of successful reclosing of overhead line reclosing in AC systems is 60% to 90%, and as the voltage level increases, the recombination power also increases substantially. Therefore, the reclosing technology can greatly improve the AC system's ability to respond to line faults, and the technology can also be applied to flexible DC grids that transmit electricity through overhead lines. However, the reclosing technology also has certain disadvantages. Since the circuit breaker may coincide with a permanent fault when it recloses, the power system will suffer two consecutive fault shocks in a short period of time, which will reduce the insulation of electrical equipment and reduce its service life. For the flexible DC grid, due to the low impedance characteristics of the flexible DC grid and the fragility of power electronic devices, the damage caused by continuous fault impact is far greater than the AC system. Therefore, in order to avoid coincidence in permanent faults and to provide a basis for DC circuit breaker whether to perform coincidence or not, it is of great significance to study the identification methods of fault properties suitable for flexible DC power grids.

At present, the AC system mainly uses the recovery voltage generated by the coupling between the fault removal phase and the healthy phase to identify instantaneous and permanent faults. Since there is no coupling relationship between the pole lines after the fault removal of the flexible DC grid, this method is not applicable to the flexible DC system.

Summary of the invention

In order to solve the above problems, the present invention proposes an auxiliary circuit of a hybrid DC circuit breaker, a method and system for identifying fault properties of a multi-terminal flexible DC grid. The auxiliary circuit of the hybrid DC circuit breaker injects characteristic signals into the DC line to achieve DC line failure. Nature discrimination. By adding an auxiliary circuit to the hybrid DC circuit breaker, the energy stored in the buffer capacitor of the IGBT sub-module after the hybrid DC circuit breaker cuts off the fault current can be used to determine the nature of the fault. The additional cost of hardware is small, the control is simple, and it has permanent fault detection. Distance function. It is of great significance to improve the power supply reliability of DC overhead lines and avoid secondary shocks caused by overlapping permanent faults.

In order to achieve the above objectives, the present invention adopts the following technical solutions:

An auxiliary circuit of a hybrid DC circuit breaker disclosed in one or more embodiments, the auxiliary circuit is added to the Nc IGBT submodules on the side of the hybrid DC circuit breaker transfer branch close to the overhead line; the auxiliary circuit The circuit includes: a thyristor, a resistor R _g, and a fast mechanical switch. The thyristor is respectively connected in series between the overhead line and the negative electrode of the buffer capacitor of the buffer circuit of the first IGBT submodule and between the buffer capacitors of the buffer circuits of two adjacent IGBT submodules. In the meantime, the positive electrode of the buffer capacitor of the buffer circuit of the Ncth IGBT sub-module is connected in series with a resistance and a fast mechanical switch before being grounded; among them, Nc is the set value.

Further, the N _{c is} taken as 10%-20% of the total number N of the transfer branch IGBT sub-modules.

A multi-terminal flexible DC grid fault identification system configured with a hybrid DC circuit breaker disclosed in one or more embodiments includes the above-mentioned auxiliary circuit of the hybrid DC circuit breaker, which will be stored in the converter branch IGBT sub-module The energy in the snubber capacitor of the snubber circuit is used as a characteristic signal injection source to detect the fault nature of the DC line.

A method for identifying fault properties of a multi-terminal flexible DC grid equipped with a hybrid DC circuit breaker disclosed in one or more embodiments is characterized in that the method is based on the above-mentioned auxiliary circuit and specifically includes:

After the hybrid DC circuit breaker cuts off the fault current, after a set time of fault point arc de-free time, trigger signals are applied to all the thyristors in the auxiliary circuit, and the fast mechanical switch in the auxiliary circuit is closed;

Real-time collection of current data on the line side of the hybrid DC circuit breaker;

Perform wavelet transform on the collected current data, and calculate the first-scale wavelet transform modulus maximum;

Perform data validity verification on the obtained wavelet transform modulus maximum value, and record the first and second wavelet transform modulus maximum value symbols and sampling times that meet the data validity conditions;

Establish the criterion for identifying the nature of the fault, and use the calculated wavelet transform modulus maximum symbol to identify the transient fault and the permanent fault.

Further, if it is recognized as a transient fault, open the fast mechanical switch in the auxiliary circuit and remove the trigger signal of the thyristor, and the hybrid DC circuit breaker will reclose; if it is recognized as a permanent fault, open the fast mechanical switch in the auxiliary circuit Switch and remove the trigger signal of the thyristor, and the hybrid DC circuit breaker does not reclose.

Further, the fault distance is calculated according to the obtained first and second wavelet transform modulus maximum sampling moments that satisfy the data validity condition.

Further, the wavelet transform modulus maximum that satisfies the data validity condition is specifically:

Set the setting value WTMM _set , and take the data whose absolute value of the wavelet transform modulus maximum value is greater than the setting value as the wavelet transform modulus maximum value that meets the data validity condition;

Among them, the setting value WTMM _{set is} determined according to the maximum value of the wavelet transform modulus maximum value that can be caused by noise.

Further, establish the criteria for identifying the nature of the fault, specifically:

Define the first and second wavelet transform modulus maxima that meet the data validity conditions at the first scale as WTMM _f and WTMM _s , if they meet:

WTMM _f ·WTMM _s >0;

It is judged as a permanent fault, otherwise it is judged as a transient fault.

A multi-terminal flexible DC power grid fault identification system configured with a hybrid DC circuit breaker disclosed in one or more embodiments includes a server. The server includes a memory, a processor, and is stored on the memory and can be stored on the processor. A running computer program, when the processor executes the program, the above-mentioned method for identifying the nature of a fault in a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker is realized.

A computer-readable storage medium disclosed in one or more embodiments has a computer program stored thereon, and when the program is executed by a processor, the above-mentioned multi-terminal flexible DC power grid fault identification method configured with a hybrid DC circuit breaker is executed .

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

(1) Use the auxiliary circuit of the hybrid DC circuit breaker to realize the injection of characteristic signals to identify instantaneous and permanent faults, and realize the fault location of permanent faults;

(2) Reliably and quickly identify instantaneous and permanent faults under various initial conditions of faults. Factors such as transition resistance, fault location, and fault type have almost no effect on the identification results of the fault nature, with high reliability and sensitivity;

(3) The fault identification method is based on the polarity of the current reflected wave, which has absolute selectivity;

(4) The principle of the identification method is simple and clear, the identification is accurate, easy to implement in engineering, the additional cost of auxiliary circuit is small, and the accuracy of permanent fault location can be achieved by increasing the sampling rate, which has high practical value.

BRIEF DESCRIPTION

The drawings of the specification forming a part of the application are used to provide a further understanding of the application, and the exemplary embodiments and descriptions of the application are used to explain the application, and do not constitute an improper limitation of the application.

Figure 1 is a schematic diagram of the structure of the hybrid DC circuit breaker in the first embodiment;

2 is a schematic diagram of the structure of a diode full-bridge IGBT sub-module in the first embodiment;

3 is a schematic diagram of the charging and discharging path of the buffer capacitor of the IGBT sub-module in the first embodiment;

Figure 4 (a)-(b) are the transient current and voltage waveforms of each branch of the hybrid DC circuit breaker in the first embodiment;

Figure 5 (a)-(b) are the transient current waveforms of the hybrid DC circuit breaker in the first embodiment and the second commutation process;

6 is a schematic diagram of the auxiliary circuit structure of the hybrid DC circuit breaker in the first embodiment;

7 is a schematic diagram of the discharge path of the buffer capacitors of N _c IGBT sub-modules in the second embodiment;

8 is an equivalent circuit of the discharge process of the buffer capacitors of N _c IGBT sub-modules in the second embodiment;

Fig. 9 is a grid diagram of current traveling wave refracted reflection in the second embodiment;

Figure 10 is a flowchart of a method for identifying the nature of a fault in the second embodiment;

Figure 11 is a simulation model of the dual-terminal MMC-HVDC system in the second embodiment;

Figure 12 is a configuration diagram of a DC overhead line in the second embodiment;

Figure 13(a)-(b) are respectively the DCCB1 transient current and wavelet transform modulus maximum waveform under permanent fault in the second embodiment;

Figure 14(a)-(b) are respectively the DCCB1 transient current and wavelet transform modulus maximum waveform under transient fault in the second embodiment;

Figure 15(a)-(b) are the identification results of the fault nature at different fault locations in the second embodiment;

Figure 16(a)-(b) are respectively the DCCB1 transient current and wavelet transform modulus maximum waveform under the permanent inter-electrode short-circuit fault in the second embodiment;

Figures 17(a)-(b) respectively show the DCCB1 transient current and wavelet transform modulus maximum waveforms under permanent faults with different transition resistances in the second embodiment.

detailed description

It should be pointed out that the following detailed descriptions are all exemplary and are intended to provide further descriptions of this application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs.

It should be noted that the terms used here are only for describing specific implementations, and are not intended to limit the exemplary implementations according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "comprising" and/or "including" are used in this specification, they indicate There are features, steps, operations, devices, components, and/or combinations thereof.

Example one

In one or more embodiments, an auxiliary circuit of a hybrid DC circuit breaker is disclosed. As shown in FIG. 6, the auxiliary circuit is added to the Nc IGBT sub-modules on the side of the overhead line near the transfer branch of the hybrid DC circuit breaker. The auxiliary circuit includes: a thyristor, a resistor Rg and a fast mechanical switch, the thyristor is respectively connected in series between the overhead line and the buffer capacitor negative electrode of the first IGBT sub-module buffer circuit and the buffer capacitors of two adjacent IGBT sub-module buffer circuits In between, the positive electrode of the buffer capacitor of the buffer circuit of the Nc-th IGBT sub-module is connected in series with a resistance and a fast mechanical switch before being grounded; where Nc is the set value.

The hybrid DC circuit breaker is composed of the main branch, the transfer branch and the metal oxide varistor (MOV), as shown in Figure 1. The main branch routing is composed of IGBT-based load current switches (LCS) and ultra-high-speed mechanical switches in series, and LCS is mainly composed of IGBT sub-modules in series and parallel. The transfer branch is composed of many IGBT sub-modules in series (usually hundreds in high-voltage and large-capacity application scenarios), and is divided into several groups, each group is connected with a MOV in parallel, and the MOV is used to absorb the residual energy in the fault circuit. The IGBT sub-modules in the transfer branch mainly have IGBT full-bridge, diode full-bridge, co-emission, collector series and other structures. These structures are combined to use IGBT current transfer capabilities and capacitive current buffering capabilities to achieve circuit breaking The converter successfully commutates. Due to the lower cost of the diode full-bridge type, diode full-bridge IGBT sub-modules are often used in high-voltage and large-capacity situations. The structure is shown in Figure 2.

In Fig. 2, R _s represents the snubber resistance, C _s represents the snubber capacitor, and D _s represents the diode. The above three elements together constitute the snubber circuit in the IGBT sub-module. When the IGBT is switched from on to off, the current flowing in the IGBT is transferred to the buffer capacitor of the sub-module, and the buffer capacitor is charged through the diode. In this process, the buffer resistor is short-circuited by the diode. When the charging process is over, the voltage on the snubber capacitor and the IGBT is mainly determined by the external circuit, and since the snubber capacitor has no discharge circuit when the IGBT is off, the voltage on the snubber capacitor can still maintain a high level when the external circuit voltage drops . When the IGBT is switched from the off state to the on state, the snubber capacitor in the snubber circuit will discharge through the snubber resistor. The charging and discharging path of the diode full-bridge IGBT sub-module is shown in Figure 3.

When the DC transmission system is operating normally, the load current all passes through the main branch of the hybrid DC circuit breaker. Since the main branch mainly contains mechanical switches and a small amount of IGBT sub-modules in the load current switch, the loss is small, usually 0.01% The delivery capacity. When a fault occurs, the fault current is commutated from the main branch to the transfer branch by blocking the IGBT sub-module in the LCS, and the ultra-high-speed mechanical switch is disconnected when the fault current in the main branch weakens to the off-condition of the ultra-high-speed mechanical switch , At this time the fault current has all been commutated to the transfer branch. After that, the IGBT sub-module of the transfer branch is blocked, the fault current is commutated to the MOV branch, and the residual energy stored in the DC reactor and the fault circuit is consumed through the MOV, thereby cutting off the fault current. When the hybrid DC circuit breaker breaks the fault current, the current in each branch of the circuit breaker (i _main represents the main branch current, i _transfer represents the current in the commutation branch, i _MOV represents the current in the MOV) and voltage as shown in Figure 4 Shown.

In Figure 4, when a fault occurs at t ₀ , the main branch current rises rapidly; at t ₁ the fault current flowing through the main branch reaches the maximum switchable current or the DC circuit breaker receives a trip signal, the IGBT in the LCS is blocked and the fault occurs The current starts to commutate to the transfer branch, and the first commutation process begins; the first commutation is completed at t ₂ and the ultra-high-speed mechanical switch starts to open; the ultra-high-speed mechanical switch is opened at t ₃ and the fault occurs at this time The current has been fully commutated to the transfer branch. At this time, the transfer branch IGBT sub-module starts to block the current, the current is commutated to the MOV branch, and the second commutation starts; the second commutation is completed at t ₄ , and then the current is at The MOV begins to drop; the current drops to zero at t ₅ , the hybrid DC circuit breaker is completed, and the fault current is cleared.

In the above process, due to the existence of the snubber capacitor of the IGBT sub-module, the first detailed commutation process in the time period t ₁ -t ₂ is that the fault current is firstly commutated from the main branch IGBT to the main branch snubber capacitor, and then from The snubber capacitor of the main branch commutates to the transfer branch IGBT. Similarly, during the second commutation process in the time period t ₃ -t ₄ , the fault current is first commutated from the transfer branch IGBT to the transfer branch buffer capacitor, and then from the transfer branch buffer capacitor to the MOV. The transient current waveform during the two commutation processes is shown in Figure 5.

After the operation of the hybrid DC circuit breaker is completed, the energy stored in the snubber capacitor has not disappeared. Therefore, by adding an auxiliary circuit, the energy of the snubber capacitor stored in the IGBT sub-module of the converter branch can be used as a characteristic signal injection source. Detect the nature of the fault in the DC line.

The auxiliary circuit of the hybrid DC circuit breaker for identifying the nature of the fault is shown in Figure 6. The auxiliary circuit mainly includes a thyristor, a resistance R _g and a fast mechanical switch. The auxiliary circuit is added to the N _c IGBT sub-modules near the line side of the transfer branch of the hybrid DC circuit breaker, where N _{c is} recommended to be taken as 10%-20% of the total number N of the transfer branch IGBT sub-modules. The number of N _c can affect the intensity of the injected signal.

Example 2

Based on the auxiliary circuit described in the first embodiment, a method for identifying the nature of a fault in a multi-terminal flexible DC grid equipped with a hybrid DC circuit breaker and its application is proposed, as shown in Figure 10, including the following steps:

(1) Configure an auxiliary circuit for the hybrid DC circuit breaker. The auxiliary circuit includes a thyristor, a resistor R _g and a fast mechanical switch. The structure is shown in Figure 6;

(2) After the hybrid DC circuit breaker cuts off the fault current, after 200 milliseconds of fault point arc de-free time, trigger signals are first applied to all the thyristors in the auxiliary circuit, and then the fast mechanical switch in the auxiliary circuit is closed;

(3) Real-time collection of current data on the line side of the hybrid DC circuit breaker;

(4) Perform wavelet transform on the current data collected in step (3) to calculate the first-scale wavelet transform modulus maximum; specifically:

In the formula, a ₀ (k) is the original signal, h ₀ , h ₁ are low-pass and high-pass filters respectively; a _j (k), d _j (k) are the j-th layer smoothing approximation coefficient and detail coefficient of the original signal , Respectively represent the signal components of frequency bands 0～f _s /2 ^j+1 and f _s /2 ^j+1 ～f _s /2 ^j extracted from the original signal with sampling rate f _s ;

Wavelet transform modulus maximum value d _j (k _n ) is defined as: |d _j (k _n )|≥|d _j (k)|;

Among them, d _j (k) is the smooth detail coefficient of the j-th layer of the original signal.

(5) Perform a data validity check on the wavelet transform modulus maximum value obtained in step (4), and record the first and second wavelet transform modulus maximum value symbols and sampling times that meet the data validity conditions; specifically :

Set the setting value WTMM _set , and take the data whose absolute value of the wavelet transform modulus maximum value is greater than the setting value as valid data;

In the formula, the setting value WTMM _{set is} determined according to the maximum value of the wavelet transform modulus maximum value that can be caused by noise.

(6) Establish a criterion for identifying the nature of the fault, and use the wavelet transform modulus maximum symbol calculated in step (5) to identify transient and permanent faults;

Define the first and second wavelet transform modulus maxima that meet the data validity conditions at the first scale as WTMM _f , WTMM _s , if they meet

WTMM _f ·WTMM _s >0;

It is judged as a permanent fault, otherwise it is judged as a transient fault.

(7) If step (6) is identified as a transient fault, turn on the fast mechanical switch in the auxiliary circuit and remove the trigger signal of the thyristor, and then the hybrid DC circuit breaker will reclose. If step (6) is identified as a permanent fault, the fast mechanical switch in the auxiliary circuit is opened and the trigger signal of the thyristor is removed, and the hybrid DC circuit breaker does not reclose. At the same time, the fault distance is calculated according to the first and second wavelet transform modulus maximum sampling moments obtained in step (5).

In step (6), the principle of identifying the nature of the fault is:

After the trigger signal is applied by the thyristor, the fast mechanical switch is closed and the snubber capacitors of the N _c IGBT sub-modules on the line side of the transfer branch are discharged to identify the nature of the fault.

After the hybrid DC circuit breaker cuts off the fault current, after 200 milliseconds of fault point arcing time, a trigger signal is applied to the thyristor in the auxiliary circuit, and then the fast mechanical switch is closed. At this time, the DC bus voltage will be evenly distributed to the NN _c IGBT sub-modules near the bus side in the transfer branch of the hybrid DC circuit breaker, so the voltage of the NN _c IGBT sub-modules will rise to a new steady-state voltage. For the N _c IGBT sub-modules close to the line side in the transfer branch, when the fast mechanical switch is closed, the buffer capacitors of the N _c IGBT sub-modules will discharge the line through the thyristor, the resistance R _g and the fast mechanical switch. The discharge path is as follows As shown in Figure 7, the equivalent circuit of the discharge process is shown in Figure 8. In Figure 8, N _c U _c and C _s /N _c are the equivalent voltage and equivalent snubber capacitance of N _c IGBT sub-modules respectively, and Z _c is the wave impedance of the DC line. The on-state resistance of the thyristor is very Is small, so ignore it.

Closing the fast mechanical switch after the thyristor is triggered is equivalent to superimposing a step input with an amplitude of N _c U _c . Under the action of this step input, a current traveling wave propagating from the hybrid DC circuit breaker to the opposite end of the line will be generated. The initial current traveling wave is as follows:

In the formula, ΔI(s) and Δi(t) are the frequency domain and time domain expressions of the initial current traveling wave, respectively. The amplitude of the initial current traveling wave can be changed by changing the resistance of the resistor R _g .

Figure 9 shows the refraction and reflection grid of the current traveling wave. Refraction and reflection occur when the current traveling wave propagates to the discontinuous wave impedance. Regardless of the line attenuation, the first reflected wave of the initial current traveling wave is as follows:

ΔI _r =ΓΔI (3)

In the formula, Г is the reflection coefficient of the current traveling wave, which can be obtained by the following formula:

In the formula, Z ₂ is the equivalent characteristic impedance of the part where the current traveling wave will enter, and Z _c is the wave impedance of the line.

Assuming that the transition resistance at the fault point is R _f , the current traveling wave reflection coefficient Г _f at the fault point is:

It can be seen from the above formula that, regardless of the value of the transition resistance, Z ₂ is less than Z _c . Therefore, the reflection coefficient Γ _{f of the} current traveling wave at the fault point is always greater than zero.

For the current traveling wave reflection coefficient Γ _t at the opposite end of the line, since the DC line is isolated, that is, the opposite end of the line is open, Z ₂ is infinite at this time, so the current traveling wave reflection coefficient Γ _t at the opposite line port is approximately -1.

Based on the above analysis, we can draw conclusions:

After the fast mechanical switch is closed, if the initial current traveling wave detected on the line side of the DC circuit breaker has the same polarity as the first reflected wave of the initial current traveling wave, it means that the fault still exists on the line, otherwise the fault has disappeared. Because if the fault still exists when the fast mechanical switch is closed, the first reflected wave of the initial current traveling wave detected by the relay is the reflected wave at the fault point. Since the reflection coefficient of the current traveling wave at the fault point is positive, the first reflection is The wave has the same polarity as the initial current traveling wave. Otherwise, the first reflected wave of the initial current traveling wave detected by the relay is the line opposite end reflection wave, and the line opposite end current traveling wave reflection coefficient is negative, so the first reflection wave has the opposite polarity to the initial current traveling wave.

Therefore, based on the above conclusions, it can be judged whether the fault on the DC line is a transient fault or a permanent fault.

In step (7), when it is determined that a permanent fault has occurred, the fault distance calculation method is:

It can be seen from Figure 9 that the fault distance can be calculated using the following formula:

In the formula, v is the propagation speed of the current traveling wave, t _f is the detected initial current traveling wave moment, and t _s is the arrival time of the first reflected wave. If the DC line fault has disappeared after 200 milliseconds of dissociation time, t _s is the arrival time of the reflected wave at the opposite end of the line, and l _m is equal to the line length at this time. The arrival time of the reflected wave can be accurately obtained by the corresponding time of the wavelet transform modulus maximum.

The calculation accuracy of the above formula is related to the sampling rate. If the sampling rate is f, the maximum ranging error l _error of the above formula is:

Use PSCAD to build a simulation model of the dual-terminal MMC-HVDC flexible DC system, and simulate and verify the proposed fault identification method:

1) Build a model

According to the relevant parameters of the Zhangbei Flexible DC Demonstration Project, the present invention builds a ±500kV double-ended flexible straight system simulation model shown in FIG. 11 in the PSCAD/EMTDC software. The converter station in Figure 11 has a symmetrical bipolar structure, and each converter station contains two MMCs. The main parameters of the converter station are shown in Table 1. In the simulation model, the DC overhead line adopts a distributed frequency-dependent model, and its length is 262km, and its configuration is shown in Figure 12.

Table 1 Main parameters of converter station

The main parameters of the hybrid DC circuit breaker are shown in Table 2. According to the line parameter program of PSCAD/EMTDC software, the propagation speed of the current traveling wave can be obtained as 298.41 m/microsecond, and the wave impedance of the DC line is 325Ω. The sampling frequency of the DC circuit breaker port relay is 100kHz, so according to formula (7), the maximum error of fault location can be obtained as 1.492km. In the simulation model, N _{c is} taken as 10% of the total number of IGBT sub-modules of the transfer branch, which is 30. Therefore, according to the schematic diagram of the auxiliary circuit structure shown in FIG. 6, the required number of thyristors is also 30. In order to limit the initial current traveling wave amplitude of the outlet fault of the DC circuit breaker to less than 1kA, the resistance R _{g is} taken as 80Ω.

Table 2 Main parameters of hybrid DC circuit breaker

Since the transient fault duration is usually less than 200 milliseconds, after the DCCB interrupts the fault current, the identification of the nature of the fault starts at 200 milliseconds. To avoid noise interference, WTMM _{set is} set to 0.005.

2) Permanent fault simulation

S converter station at a distance of ₁ 100km provided at a permanent positive ground fault, fault resistance is 50Ω. The identification of the nature of the fault starts 200 milliseconds after the hybrid DC circuit breaker cuts off the fault current. The detected current and its wavelet transform modulus maximum value are shown in Figure 13(a)-(b).

From Figure 13(a) and Figure 13(b), it can be seen that the first and second wavelet transform modulus maximums that meet the data validity conditions are both negative, so the fault is judged to be a permanent fault. The circuit breaker no longer recloses. In addition, the calculated fault distance is 99.967km, and the distance error from the actual fault is only 0.033km.

3) Transient fault simulation

S converter station at a distance of ₁ 100km disposed at a positive transient earth fault, the fault duration is 200 ms, the transition resistance is 50Ω. The identification of the nature of the fault starts 200 milliseconds after the hybrid DC circuit breaker cuts off the fault current. The relevant waveforms are shown in Figure 14 (a) and Figure 14 (b).

Since the line fault is a transient fault, when the fault nature is identified, the fault has disappeared. The first reflected current traveling wave detected by the outlet relay of the DC circuit breaker is reflected by the opposite end of the line instead of the fault point. As shown in Figure 14(a) and Figure 14(b), the first and second wavelet transform modulus maxima that meet the data validity conditions have different signs. Therefore, the fault identification result is a transient fault. The DC circuit breaker can be reclosed.

4) Analysis of influencing factors

The influencing factors considered in this section are fault location, fault distance, and fault type.

A. Different fault locations. A positive electrode disposed permanent ground fault on the dc line from the converter station S ₁ to 26.2km (10% line length) of each interval, the transition resistance 50Ω. The identification of the nature of the fault starts 200 milliseconds after the DC circuit breaker cuts off the fault current. The relevant waveforms are shown in Figure 15(a) and Figure 15(b).

It can be seen from Figure 15(a) and Figure 15(b) that the first and second wavelet transform modulus maxima that meet the data validity condition have the same sign, so the fault on the line is judged to be a permanent fault. Therefore, the fault identification result is correct, and the maximum error of fault location is 0.3%, which can meet engineering needs. Therefore, the location of the fault will not affect the results of the fault identification method.

B. Different failure types. S converter station at a distance of ₁ 100km disposed at the inter-pole permanent short-circuit fault, the fault resistance is 50Ω. The identification of the nature of the fault starts 200 milliseconds after the hybrid DC circuit breaker cuts off the fault current. The results are shown in Figure 16 (a) and Figure 16 (b).

As shown in Figure 16(a) and Figure 16(b), the first and second wavelet transform modulus maxima that meet the data validity conditions are both negative, so the fault identification method determines that the fault is permanent malfunction. The fault location result is 99.067km, and the error is only 0.033km. The result is the same as the fault location result of the positive ground fault at the same fault location. Therefore, the fault type has almost no influence on the identification results of the fault nature identification method.

C. Different transition resistance. S converter station at a distance of ₁ 100km provided at a permanent positive ground fault, fault resistance, respectively 0.01Ω, 50Ω, 250Ω and 500Ω. The identification results of the fault nature are shown in Figure 17(a) and Figure 17(b).

It can be seen from Figure 17(a) and Figure 17(b) that changing the transition resistance does not affect the polarity and arrival time of the reflected wave, but only affects the amplitude of the current traveling wave. However, even if the transition resistance is as high as 500Ω, the fault identification method can still accurately determine the fault nature and calculate the fault distance.

D. Other influencing factors. According to formula (1), the amplitude of each frequency component of the initial current traveling wave |ΔI(jω)| is:

Therefore, without considering the line attenuation, the amplitude of each frequency component of the reflected wave of the initial current traveling wave at the fault point |ΔI _r (jω)| is:

Where Г is the reflection coefficient of the current traveling wave at the fault point, which can be approximately regarded as a constant.

It can be seen from equation (9) that the amplitude of each frequency component of the reflected wave at the fault point |ΔI _r (jω)| can be increased by increasing the buffer capacitance C _s and the number of IGBT sub-modules N _c used, thereby increasing The sensitivity of the fault identification method.

The invention realizes the identification of the nature of the fault by arranging auxiliary circuits for the hybrid DC circuit breaker, and using the hybrid DC circuit breaker to cut off the fault current to transfer the energy stored in the buffer capacitor of the branch circuit. First apply the trigger signal to the thyristor in the auxiliary circuit, then close the fast mechanical switch, use the current data collected at the exit of the hybrid DC circuit breaker and perform wavelet transformation to obtain the first scale of the current traveling wave that satisfies the data validity condition. One and the second wavelet transform modulus maximum value, and then identify the instantaneous and permanent faults through its polarity, and calculate the fault distance of the permanent fault after the permanent fault is judged. Simulation analysis shows that under various fault conditions, the present invention can correctly identify transient faults and permanent faults, with high sensitivity and strong reliability. In addition, the invention can also realize permanent fault location, with small ranging error and high accuracy.

Although the specific embodiments of the present invention are described above in conjunction with the accompanying drawings, they do not limit the scope of protection of the present invention. Those skilled in the art should understand that on the basis of the technical solutions of the present invention, those skilled in the art do not need to make creative efforts. Various modifications or variations that can be made are still within the protection scope of the present invention.

Claims

An auxiliary circuit of a hybrid DC circuit breaker, characterized in that the auxiliary circuit is added to Nc IGBT submodules on the side of the transfer branch of the hybrid DC circuit breaker close to the overhead line; the auxiliary circuit includes: a thyristor, a resistor R g and a fast mechanical switch, the thyristors are respectively connected in series between the overhead line and the negative electrode of the buffer capacitor of the first IGBT sub-module buffer circuit and between the buffer capacitors of two adjacent IGBT sub-module buffer circuits. The Ncth IGBT The positive electrode of the buffer capacitor of the sub-module buffer circuit is connected in series with a resistance Rg and a fast mechanical switch before being grounded; among them, Nc is the set value.
The auxiliary circuit of a hybrid DC circuit breaker according to claim 1, wherein the N c is taken as 10%-20% of the total number N of the IGBT sub-modules of the transfer branch.
A multi-terminal flexible DC power grid fault identification system equipped with a hybrid DC circuit breaker, which is characterized in that it comprises the auxiliary circuit of the hybrid DC circuit breaker according to any one of claims 1-2, which is stored in the transfer branch IGBT The energy in the buffer capacitor of the sub-module buffer circuit is used as a characteristic signal injection source to detect the fault nature of the DC line.
A method for identifying fault properties of a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker is characterized in that the method is based on the auxiliary circuit according to any one of claims 1-2, and specifically includes:

After the hybrid DC circuit breaker cuts off the fault current, after a set time of fault point arc de-free time, trigger signals are applied to all the thyristors in the auxiliary circuit, and the fast mechanical switch in the auxiliary circuit is closed;

Real-time collection of current data on the line side of the hybrid DC circuit breaker;

Perform wavelet transform on the collected current data, and calculate the first-scale wavelet transform modulus maximum;

Perform data validity verification on the obtained wavelet transform modulus maximum value, and record the first and second wavelet transform modulus maximum value symbols and sampling times that meet the data validity conditions;

Establish the criterion for identifying the nature of the fault, and use the calculated wavelet transform modulus maximum symbol to identify the transient fault and the permanent fault.
A method for identifying the nature of a fault in a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker according to claim 4, wherein if it is identified as a transient fault, the fast mechanical switch in the auxiliary circuit is turned on and the triggering of the thyristor is removed Signal, the hybrid DC circuit breaker performs reclosing; if it is recognized as a permanent fault, the fast mechanical switch in the auxiliary circuit is opened and the trigger signal of the thyristor is removed, and the hybrid DC circuit breaker does not perform reclosing.
The method for identifying the fault properties of a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker according to claim 4, characterized in that, according to the obtained first and second wavelet transform modulus maxima that satisfy the data validity condition Calculate the fault distance at the time of value sampling.
A method for identifying fault properties of a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker according to claim 4, wherein the wavelet transform modulus maximum value that meets the data validity condition is specifically:

Set the setting value WTMM set , and take the data whose absolute value of the wavelet transform modulus maximum value is greater than the setting value as the wavelet transform modulus maximum value that meets the data validity condition;

Among them, the setting value WTMM set is determined according to the maximum value of the wavelet transform modulus maximum value that can be caused by noise.
The method for identifying the nature of a fault in a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker according to claim 4, wherein the establishment of a criterion for identifying the nature of the fault is specifically:

Define the first and second wavelet transform modulus maxima that meet the data validity conditions at the first scale as WTMM f and WTMM s , if they meet:

WTMM f ·WTMM s >0;

It is judged as a permanent fault, otherwise it is judged as a transient fault.
A multi-terminal flexible DC power grid fault identification system configured with a hybrid DC circuit breaker is characterized by comprising a server, the server including a memory, a processor, and a computer program stored in the memory and running on the processor. When the processor executes the program, the method for identifying the fault properties of a multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker according to any one of claims 4-8 is realized.
A computer-readable storage medium with a computer program stored thereon, wherein the program executes the multi-terminal flexible DC power grid equipped with a hybrid DC circuit breaker according to any one of claims 4-8 when the program is executed by a processor Nature identification method.