METHOD FOR PROTECTING DC LINE IMPEDANCE PHASE BASED ON PROTECTION AND CONTROL COORDINATION

Abstract

The present disclosure relates to a method for protecting DC line impedance phase based on protection and control coordination, and an application scenario of the method for protecting is a three-terminal flexible DC transmission network. The method uses high controllability of a converter after a fault, injects a characteristic signal at a characteristic frequency, and calculates a phase angle of input impedance to determine a fault interval, which effectively improves protection performance, turns passive to active, and is not affected by nonlinearity of the converter. At the same time, compared with a full-bridge MMC, using a half-bridge MMC does not need to perform fault ride-through first when identifying a fault, and does not need to add additional equipment, it creates fault features and can reliably identify an fault interval; improves protection quickness and at the same time also has better economic benefits. It has selectivity, and an entire system may not be shut down due to failure of a single line.

Claims

1. A method for protecting DC line impedance phase based on protection and control coordination, an application scenario of the method for protecting being a three-terminal flexible DC transmission network, comprising steps as follows: S1, after a line fails, a fault point generating a fault traveling wave, the traveling wave propagating at a speed of light to both ends of the line; S2, upon sensing a sharp change in voltage by measuring apparatuses at both the ends of the line, it is considered that a DC system has a fault, and a discriminant formula is as follows: $\left|d{u}_{Bus2}/dt\right|>\Delta U_{set}$ wherein, u.sub.Bus2 is voltage at a point U.sub.Bus2, and ΔU.sub.set is a threshold value set for detecting a fault; S3, when the fault is identified, further using a formula (4) to determine a direction of the fault, if a voltage change rate on a line side of a current-limiting reactor is greater than a voltage change rate on a converter side of the current-limiting reactor, the fault is considered as a forward fault, sending a converter active injection control start signal, or if a voltage change rate on a line side of a current-limiting reactor is less than a voltage change rate on a converter side of the current-limiting reactor, active injection control is not being activated; $\left|d{u}_{Bus2}/dt\right|>\left|d{u}_{Con2}/dt\right|$ wherein, u.sub.Con2 is voltage at a point U.sub.Con2; S4, when the formulas (3) and (4) are satisfied at the same time, switching a MMC.sub.2 converter to a control mode 2, and activating active injection control, a frequency of a generated characteristic signal being 600 Hz and lasting for at least 2 cycles; and S5, extracting a characteristic voltage and a current signal at an injection frequency, and calculating a phase angle difference between the voltage and the current; and constructing a fault identification criterion: $\angle {\dot{U}}_{in}-\angle {\dot{I}}_{in}=\theta >{\Delta }_{in}=0°$ wherein, U.sub.in is a characteristic voltage vector at the injection frequency at an input terminal, and I.sub.in is a characteristic current vector at the injection frequency at the input terminal; and if a calculated θ is greater than 0, it is considered that an internal fault occurs, or if a calculated θ is less than 0, it is considered that an external fault occurs.

2. The method for protecting DC line impedance phase based on protection and control coordination according to claim 1, wherein the three-terminal flexible DC transmission network comprises: an MMC.sub.1 converter, the MMC.sub.2 converter and an MMC.sub.3 converter; and the MMC.sub.1 converter and the MMC.sub.2 converter are power control stations, and the MMC.sub.3 converter is a voltage control station.

3. The method for protecting DC line impedance phase based on protection and control coordination according to claim 1, wherein, in step S4, the control mode 2 is active injection control.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present disclosure has the following accompanying drawings:

[0022] FIG. 1 is a schematic diagram of an internal structure of a converter;

[0023] FIG. 2 is a block diagram of converter control;

[0024] FIG. 3 is a topology of a three-terminal flexible DC transmission network;

[0025] FIG. 4 is a distribution parameter model of a transmission line;

[0026] FIG. 5 is a characteristic diagram of an input impedance phase angle under an external fault;

[0027] FIG. 6 is a characteristic diagram of an input impedance phase angle under an internal fault;

[0028] FIG. 7 is a block diagram of converter control;

[0029] FIG. 8 is a characteristic diagram of an internal fault; and

[0030] FIG. 9 is a characteristic diagram of an external fault.

DETAILED DESCRIPTION OF EMBODIMENTS

[0031] The present disclosure will be further described in detail below with reference to the accompanying drawings.

1. Converter Control Mode

[0032] A main topology of MMC is as shown in FIG. 1. The interior of a converter is composed of three bridge arms, where L.sub.arm is inductance of upper and lower bridge arms, R.sub.arm is bridge arm resistance, L.sub.dc is equivalent inductance of a DC side, and R.sub.dc is equivalent resistance of the DC side. In steady-state operation before a fault, a sub-module capacitor C.sub.arm is turned on or bypassed by turning on or off IGBT elements T1 and T2 in a sub-module. When T1 is turned on, the sub-module capacitor is turned on and is in a state of charging or discharging; when T2 is turned on, the sub-module capacitor is bypassed. Under normal circumstances, a control link is as shown in mode 1 (double closed-loop vector control) in FIG. 2. A sum of the input number of three-phase upper and lower bridge arm sub-modules on the DC side is kept the same to maintain voltage stability, and an AC side is maintained by the number of sub-modules for upper and lower bridge arms turned on allocated by an AC reference voltage generated by double closed-loop control.

[0033] When a line fails, a fault traveling wave reaches both ends of the line and causes a sharp change in voltage and current. After sensing the fault, the converter immediately switches a control mode to control mode 2 (active injection control). By directly changing a proportion of the number of turns on each bridge arm, a sum of the number of sub-modules of the upper and lower bridge arms may change in a sinusoidal law, and at the same time meet the demand of AC side voltage in the shortest possible time. Finally, the converter may output voltage at a characteristic frequency and responds on the DC side.

2. Active Injection Protection Application Scenario

[0034] In the present disclosure, an example application scenario is a three-terminal flexible DC transmission network, as shown in FIG. 3. In this topology, the first two of MMC.sub.1, MMC.sub.2 and MMC.sub.3 are power control stations, and the latter is a voltage control station. A length of a line L.sub.12 and a line L.sub.23 is 100 km. When the DC system fails, the converter station MMC.sub.2 will be switched to an active injection control mode, and outlet voltage will be sinusoidal.

1) Amplitude of Injected Signal

[0035] In a normal modulation mode, a sum of the number of sub-modules of the upper and lower bridge arms of each phase of the MMC remains unchanged, and is half of the total, in this regard, an input ratio is defined as D=0.5. After the converter is switched to mode 2 of active injection control, the input ratio then changes sinusoidally, and a corresponding input ratio change interval is [0, 0.5]. Since rated voltage on the DC side is 500 kV, an amplitude of output sinusoidal voltage of the converter after an actual fault is 250 kV.

2) Frequency of Injected Signal

[0036] According to the line length of the flexible DC grid in FIG. 3, the frequency of a signal generated by the converter may be selected. A distribution parameter model of the line is as shown in FIG. 4, where L.sub.0, R.sub.0, G.sub.0, and C.sub.0 are inductance, resistance, conductance, and capacitance per unit length of the line, respectively, L is a current-limiting reactor at an end of the line, U.sub.in and I.sub.in are characteristic voltage and current at an injection frequency at an input terminal, and U.sub.end and I.sub.end are voltage and current at the injection frequency at the end of the line.

[0037] When an external fault occurs on the line, large inductance at the end of the line is still connected to the line. In this regard, an expression of an input impedance amplitude Z.sub.in at the injection frequency is as follows:

[00004]$Z_{in}=\frac{{\dot{U}}_{in}}{{\dot{I}}_{in}}=j\frac{\omega L+Z_c\tan \left(\omega l\sqrt{L_0{C}_0}\right)}{1-\omega L\tan \left(\omega l\sqrt{L_0{C}_0}\right)}$

where, U.sub.inis a characteristic voltage vector at the injection frequency at the input terminal, I.sub.in is a characteristic current vector at the injection frequency at the input terminal, ω is an angular frequency at the injection frequency at the input terminal, Z.sub.c is wave impedance, and l is a length of the transmission line.

[0038] After bringing in the line parameters, phase-frequency characteristics of the input impedance may be obtained, as shown in FIG. 5. As the frequency increases, the phase alternates between -90 and 90 degrees. Here, ω.sub.ep1 is a first parallel resonance point, and ω.sub.es1 is a first series resonance point.

[0039] When the line fails, the large inductance at the end of the line is shortcircuited, and in this regard, an expression of the input impedance amplitude Z.sub.in at the injection frequency is as follows:

[00005]$Z_{in}=\frac{{\dot{U}}_{in}}{{\dot{I}}_{in}}=j{Z}_c\tan \left(\omega l\sqrt{L_0{C}_0}\right)$

[0040] After bringing in the line parameters, the phase-frequency characteristics of the input impedance may be obtained, as shown in FIG. 6. As the frequency increases, the phase alternates between -90 and 90 degrees. Here, ω.sub.ip1 is the first parallel resonance point, and ω.sub.is1 is the first series resonance point.

[0041] By comparing an impedance phase angle difference between the internal fault and the external fault, it may be seen that in the interval [ω.sub.ep1, ω.sub.es1], an external fault phase angle is 90 degrees, and the external fault is -90 degrees. Therefore, bringing in this line parameter, an injection frequency interval may be obtained as [459 Hz, 683 Hz]. Finally, the injection frequency is set as 600 Hz.

3. Implementation Principle of Active Injection Protection

[0042] Taking the flexible DC system in FIG. 3 as an example, it is assumed that a metallic ground fault occurs, to illustrate an implementation process and working principle of active injection protection. A protection flow of the present disclosure is as shown in FIG. 7:

1) Phase 1

[0043] after a line fails, a fault point generating a fault traveling wave, the traveling wave propagating at a speed of light to both ends of the line.

2) Phase 2

[0044] upon sensing a sharp change in voltage by measuring apparatuses at both the ends of the line, it is considered that a DC system has a fault, and a discriminant formula is as follows:

[00006]$\left|d{u}_{Bus2}/dt\right|>\Delta U_{set}$

where, .sup.uBus2 is voltage at a point .sup.UBus2, and .sup.ΔUset is a set threshold value.

3) Phase 3

[0045] when the fault is identified, further using a formula (4) to determine a direction of the fault, if a voltage change rate on a line side of a current-limiting reactor is greater than a voltage change rate on a converter side of the current-limiting reactor, the fault is considered as a forward fault, sending a converter active injection control start signal, or if a voltage change rate on a line side of a current-limiting reactor is less than a voltage change rate on a converter side of the current-limiting reactor, active injection control is not being activated;

[00007]$\left|d{u}_{Bus2}/dt\right|>\left|d{u}_{Con2}/dt\right|$

where, .sup.uCon2 is voltage at a point U.sub.Con2.

4) Phase 4

[0046] when the formulas (3) and (4) are satisfied at the same time, switching a MMC.sub.2 converter to a control mode 2, and activating active injection control, a frequency of a generated characteristic signal being 600 Hz and lasting for at least 2 cycles.

5) Phase 5

[0047] extracting a characteristic voltage and a current signal at an injection frequency, and calculating a phase angle difference between the voltage and the current; and constructing a fault identification criterion:

[00008]$\angle {\dot{U}}_{in}-\angle {\dot{I}}_{in}=\theta >{\Delta }_{in}=0°$

if a calculated angle θ is greater than 0, it is considered that an internal fault occurs, or if a calculated angle θ is less than 0, it is considered that an external fault occurs.

4. Fault Case Simulation Experiment

1) Internal Fault

[0048] It is set that a metallic ground fault ƒ.sub.1 occurs at 1 s, and its fault features are as shown in FIG. 8. When the traveling wave reaches a line port, a measuring element senses a rapid drop in voltage, and both of the formulas (3) and (4) are satisfied. The MMC.sub.2 converter switches the control mode to mode 2, and actively controls the switching of bridge arm sub-modules to generate a sinusoidal characteristic signal designed in advance. By measuring voltage at an extraction outlet and a phase angle of the current at the injection frequency, it can be known that an initial phase angle of the voltage U.sub.bus2 is 3.3°, an initial phase angle of the current I.sub.M is -88.2°, and an initial phase angle of the current I.sub.N is 93.1°, so it can be calculated that an initial phase angle of the input impedance of the line L.sub.12 is 91.5°, and an initial phase angle of the input impedance of the line L.sub.23 is -89.8°. According to the criterion (5), it can be obtained that the line L.sub.12 has an internal fault, and there is no internal fault on the line L.sub.23, which may effectively identify the fault interval.

2) External Fault

[0049] It is set that a metallic ground fault ƒ.sub.2 occurs at 1 s, and its fault features are as shown in FIG. 9. When the traveling wave reaches the line port, the measuring element also senses a rapid drop in voltage, and both of the formulas (3) and (4) are satisfied. The MMC.sub.2 converter switches the control mode to mode 2, and actively controls the switching of bridge arm sub-modules to generate a sinusoidal characteristic signal designed in advance. By measuring the voltage at the extraction outlet and the phase angle of the current at the injection frequency, it can be known that the initial phase angle of the voltage U.sub.bus2 is -1.1°, the initial phase angle of the current I.sub.M is 89.5°, and the initial phase angle of the current I.sub.N is 89.2°, so it can be calculated that the initial phase angle of the input impedance of the line L.sub.12 is -90.6°, and the initial phase angle of the input impedance of the line L.sub.23 is -90.3°. According to the criterion (5), it can be obtained that there is no fault inside the line L.sub.12 and the line L.sub.23, and it may be determined that an external fault occurs.

[0050] The above embodiments are only used to illustrate the present disclosure, but not to limit the present disclosure. Those of ordinary skill in the relevant technical field may also make various changes and modifications without departing from the spirit and scope of the present disclosure. Therefore, all equivalent technical solutions also belong to the protection scope of the present disclosure.

[0051] Reference to the Drawings: [0052] FIG. 2 [0053] Double closed-loop vector control [0054] Circulation suppression [0055] Mode 1 [0056] Mode 2 [0057] Active injection control [0058] Nearest level approximation modulation [0059] Trigger control [0060] FIG. 5 [0061] Frequency [0062] Phase angle [0063] FIG. 6 [0064] Frequency [0065] Phase angle [0066] FIG. 7 [0067] Start [0068] Phase 1 [0069] Phase 2 [0070] Phase 3 [0071] Phase 4 [0072] Phase 5 [0073] Converter activating injection control [0074] Fixed delay [0075] Extracting an electrical quantity at the injection frequency [0076] L.sub.12 fault [0077] L.sub.23 fault [0078] FIG. 8 [0079] Initial angle [0080] Measuring voltage at Bus2 [0081] Measuring current [0082] Calculating impedance phase difference [0083] FIG. 9 [0084] Initial angle [0085] Measuring voltage at Bus2 [0086] Measuring current [0087] Calculating impedance phase difference

[0088] Contents not described in detail in this specification belong to the prior art known to those skilled in the art.