CN101252277A - A start-up method of no-load long line - Google Patents

Abstract

A method for starting a long no-load line belongs to the technical field of power transmission and distribution, and is used to solve the problem of quick start of a long no-load line in the black start process. The technical solution is: directly connect the inverter side of the networked VSC-HVDC to the unloaded long line to be started through a circuit breaker, close the circuit breaker first when starting, and then make the AC side of the VSC-HVDC inverter electrified, and by The inverter controls the line voltage so that the effective value of the voltage recovers according to the specified curve. The invention can effectively suppress the generation of power frequency overvoltage and closing operation overvoltage, improve the speed of black start, and shorten the time of big blackout.

Description

A start-up method of no-load long line

technical field

The invention relates to a method for quickly starting a long no-load line during the black start process of a power grid, and belongs to the technical field of power transmission and distribution.

Background technique

With the continuous increase of cross-regional interconnection of power systems, the scale of power grids is getting larger and larger, and the mutual influence between subsystems is becoming more and more intense. Partial failures of subsystems may cause large-scale power outages due to incorrect actions of protection or automatic devices. Although the possibility of such a serious accident is very small, once it happens, the loss will be huge and the consequences will be unimaginable. Therefore, how to ensure the rapid and effective restoration of system operation after a blackout has always been a problem that the power industry pays close attention to and urgently needs to be solved.

The recovery of the power grid after a major power outage is generally achieved through black-start. The method is to first use a starting power supply to charge the long unloaded line with no load, start the long unloaded line, and then gradually start other generators and loads, and finally Bring the entire grid back into operation. At present, during the black start process, gas turbines or hydraulic turbines with low power consumption requirements are generally used to charge the long unloaded line without load. The disadvantage of this method of starting the long unloaded line is that it will generate Operating overvoltage, and when the starting power supply is charging the no-load long line, power frequency overvoltage will be generated due to the line capacitance effect. In order to ensure the safety of the power grid, the closing overvoltage of the no-load line and the ferromagnetic resonance of the no-load transformer must be verified by strict calculations and experiments, otherwise it may cause extremely serious consequences. The existence of these problems greatly limits the operation speed of the black start scheme, and greatly prolongs the power outage time of the load.

Contents of the invention

The purpose of the present invention is to provide a no-load long line starting method that can effectively suppress power frequency overvoltage and closing operation overvoltage.

The said problem of the present invention is realized with following technical scheme:

A method for starting a no-load long line, which directly connects the inverter of the networked VSC-HVDC to the no-load long line XL to be started through the circuit breaker BRK. When starting, first close the circuit breaker BRK, and then make the VSC-HVDC The AC side of the inverter is charged, and the inverter controls the line voltage to restore the effective value of the voltage according to the specified curve.

In the method for starting the long line without load, the inverter controls the line voltage to recover in the following manner: within 1 to 5 power frequency cycles from the start of the long line, the effective value of the AC bus voltage at the access point of the circuit breaker BRK From 0.3 ~ 0.6p.u evenly rise to 0.8 ~ 1p.u, and ensure that the voltage at the end of the no-load line does not exceed 1.1p.u.

In the starting method of the above-mentioned no-load long line, the modulation degree m of the inverter is determined by the following formula:

$\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="cd"\; filenames="S2007101854536E00021.png"\; state="\{\{state\}\}">cd</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cq</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}}<span\; class="notranslate">\; ,</span>$

The phase angle difference between the fundamental wave component U _c of the output voltage of the converter and the fundamental wave component U _s of the AC bus voltage is determined by the following formula:

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cq</span>}}}<span\; class="notranslate">\; ;</span>$

Among them, u _cd , u _cq , are the AC side voltages in the dq0 coordinate system; U _d is the DC voltage.

In the present invention, the VSC-HVDC running on the network is used as the black start power supply in the case of a major power failure of the AC system. The "soft start" of the no-load line can be realized through the VSC-HVDC, so that the voltage of the AC system to be restored can be restored according to the specified curve, avoiding the need for AC The operation overvoltage caused by the closing of the long line and the ferromagnetic resonance caused by the empty charging of the transformer; through the independent and fast control of reactive power by VSC-HVDC, it can also avoid the power frequency overvoltage when the long line is no-load, thus greatly speeding up the blackout Boot speed. The invention can effectively suppress the generation of power frequency overvoltage and closing operation overvoltage, improve the speed of black start, and shorten the time of big blackout.

Description of drawings

The present invention will be described in further detail below in conjunction with the accompanying drawings.

Figure 1 is the electrical schematic diagram of the VSC-HVDC structure;

Fig. 2 is a system structure diagram of a voltage source converter starting a long no-load line;

Fig. 3 is a schematic diagram of the rectification side control algorithm;

Figure 4 is a control block diagram of the inverter side for suppressing power frequency overvoltage at the end of the no-load line;

Figure 5 is a block diagram of the control system on the inverter side when suppressing the overvoltage at the end of the no-load line operation;

Figure 6 shows the power frequency overvoltage simulation waveform generated at the end of the no-load line when the line length is 250km when the line is directly started and softly started;

Figure 7 shows the power frequency overvoltage simulation waveform generated at the end of the no-load line when the line length is 500km when the line is directly started and softly started;

Figure 8 is the simulation waveform of the operating overvoltage generated at the end of the no-load line when the line length is 250km and the line is directly started;

Figure 9 is the simulation waveform of the operating overvoltage generated at the end of the no-load line when the line length is 250km when the soft start is performed;

Figures 10 and 11 are the simulated waveforms of overvoltage at the moment of switching on and off after the line length is increased, before and after flexible start.

The symbols in the figure are: G, equivalent generator; R, line resistance; T ₁ , converter transformer; T ₂ , converter transformer; B ₁ , starting point of AC bus; B ₂ , end of no-load line; BRK , circuit breaker; XL, no-load line; PI, proportional integral link; PI ₁ , proportional integral link; PI ₂ , proportional integral link; AC Filter, filter circuit; VSC, voltage source converter HVDC transmission; K1, gain coefficient.

Symbols used in this paper: U _c , fundamental wave component of converter output voltage; U _s , fundamental wave component of AC bus voltage; δ, phase angle difference between U _c and U _s ; m, modulation degree; u _ca , u _cb , u _cc , converter output voltage; u _cd , u _cq , dq0 coordinate system AC side voltage; L ₀ , single-phase inductance per unit length of the line; C ₀ , single-phase capacitance per unit length of the line; R ₀ , unit length of the line Single-phase wire resistance; G ₀ , wire-to-ground leakage conductance; γ, line propagation coefficient; Z _c , line characteristic impedance; U _B1 , voltage at the starting point (B ₁ ) of the AC bus; U _B2 , voltage at the end of the line; U _B1ref , the AC bus voltage reference value at the starting point (B1) of the AC bus; Q, the reactive output of the converter; α, the imaginary part of the line propagation coefficient; β, the real part of the line propagation coefficient; U _dref , the reference value of the DC bus voltage; U _dc , the measured value of the DC bus voltage; i _1dref , i _1qref , the d-axis component of the AC current reference value on the rectification side; i _1d , i _1q , the d-axis component of the measured value of the AC current on the rectification side; i _1qref , the reference value of the AC current on the rectification side q-axis component; i _1q , q-axis component of rectification side AC current measurement value; u _s1d , u _s1q , rectification side system AC voltage d-axis and q-axis component; u _c1d , u _c1q , rectifier converter output voltage d-axis and q-axis component; Q _1ref , the reactive power reference value output by the AC system on the rectification side; pu is the unit value of the phase voltage amplitude, and U _d is the DC voltage.

Detailed ways

The present invention adopts the voltage source converter based high voltage direct current transmission technology. Compared with the traditional direct current transmission technology, the voltage source converter based high voltage direct current transmission technology (Voltage Source Converter based High Voltage Direct Current, VSC-HVDC for short) has the following main advantages:

(1) It can provide power to the passive network.

(2) It can independently and quickly control active and reactive power, and can operate in four quadrant states of active and reactive power.

(3) It can provide fast reactive power support for the AC side and play the role of STATCOM (Static Synchronous Compensator). STATCOM is an important part of the Flexible AC Transmission System (FACTS) and can realize static var compensation.

Due to the above-mentioned characteristics of VSC-HVDC, the VSC-HVDC running on the network can be used as a black-start power supply during a major power failure of the AC system. If the VSC-HVDC based on tie-line operation is used as the start-up power supply for a large blackout, many problems involved in the black start can be easily solved, and the speed of load recovery can be greatly accelerated. Through the PWM technology of VSC-HVDC, the "soft start" of VSC-HVDC is realized, so that the effective value of the voltage of the AC system to be restored increases according to the specified slope straight line or exponential function curve, avoiding the operation overvoltage and overvoltage caused by the closing of the long AC line The ferromagnetic resonance problem caused by the empty charging of the transformer; combined with the characteristics of VSC-HVDC reactive power that can be independently and quickly controlled, it can also avoid the problem of power frequency overvoltage when the long line is no-load.

The power frequency overvoltage of the no-load long line is caused by the line capacitance effect when the no-load long line is charged. The essence is that the power frequency capacitive reactance X _c of the no-load line is greater than the power frequency inductive reactance X _L. , the voltage drop U _L of the capacitive current in the line on the inductive reactance will make the voltage U _c on the capacitive reactance higher than the power supply potential at the beginning of the line.

The operating overvoltage at the end of the no-load line is generated by oscillation on the basis of its steady-state power frequency overvoltage, and finally stabilizes at its power frequency overvoltage. The higher the terminal power frequency overvoltage, the higher the amplitude of the operating overvoltage. Therefore, when the no-load line is closed, the operating overvoltage at the end of the line is largely affected by the power frequency overvoltage, and its oscillation process is very short.

From the mechanism of power frequency overvoltage, it can be seen that power frequency overvoltage is a steady-state phenomenon. Using the characteristics of VSC-HVDC inverters that can perform constant AC voltage control, when starting a long no-load line, set its initial voltage Between 0.8 and 1p.u, the power frequency overvoltage at the end of the no-load line can be suppressed. However, due to the limited number of taps for transformer adjustment and the mechanical process of adjusting voltage at the same time, the adjustment speed is very slow in the traditional method, and it cannot perform fast and high-precision control on the initial terminal voltage, so it cannot effectively suppress the power frequency overvoltage of long no-load lines. .

Since the operating overvoltage at the end of the no-load line is oscillating on the basis of steady-state power frequency overvoltage, the control involves two key points: one is to reduce the generation of electromagnetic oscillation waveforms, so the constant value of AC voltage is used to rise according to the specified curve; Second, the initial AC voltage setting is low. A reasonable recovery law should be to make the effective value of the AC bus voltage rise from 0.3 to 0.6p.u to 0.8 to 1p.u in a straight line with an exponential or a certain slope, and the duration is 1-5 cycles.

The control sequence is to suppress the operating overvoltage at the end of the no-load line first, and then suppress the power frequency overvoltage.

VSC-HVDC system structure is shown in Fig. 1. U _c is the fundamental wave component of the output voltage of the converter; U _s is the fundamental wave component of the AC bus voltage; δ is the phase angle difference between U _c and U _s ; the converter is controlled by PWM, and m is the modulation degree. Then the converter output voltages u _ca , u _cb , u _cc are:

Transform u _ca , u _cb , and u _cc through Park transformation, and get the AC side voltage u _cd and u _cq in the dq0 coordinate system as follows:

$\left[\begin{array}{c}{u}_{\mathrm{cd}}\\ u_{\mathrm{cq}}\end{array}\right]=\frac{\mathrm{<figure-callout\; id="2"\; label="m\; u\; d"\; filenames="S2007101854536E00021.png"\; state="\{\{state\}\}">m</figure-callout>}{u}_d{}_{}}{2}\left[\begin{array}{c}\sin \mathrm{\&delta;}\\ \cos \mathrm{\&delta;}\end{array}\right],$ so

$\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="u\; cd"\; filenames="S2007101854536E00021.png"\; state="\{\{state\}\}">u</figure-callout></span><figure-callout\; id="2"\; label="u\; cd"\; filenames="S2007101854536E00021.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}^{\mathrm{<span\; class="notranslate">cd</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cq</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}}<span\; class="notranslate">\; ,</span>$ $\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cq</span>}}}<span\; class="notranslate">\; ;</span>$

In the present invention, the structure of the networked VSC-HVDC starting the no-load line is shown in FIG. 2 . The process of closing the no-load line is an electromagnetic transient process, and the line is processed according to the distribution parameters. Since the actual line parameters change with the frequency, and the line is a lossy line, in order to accurately and objectively reflect its transient process, the line adopts a frequency-varying line model.

In order to simplify the analysis, when calculating the power frequency overvoltage caused by the no-load line capacitance effect, it can be assumed that the line is a lossless line and has nothing to do with frequency. The line length is 1, the single-phase inductance, capacitance, wire resistance, and ground leakage conductance of the line per unit length are L ₀ , C ₀ , R ₀ , G ₀ respectively, and the line propagation coefficient γ and characteristic impedance Z _c are:

$\mathrm{\&gamma;}=\mathrm{\&beta;}+\mathrm{j\&alpha;}=\sqrt{(R_0+\mathrm{j\&omega;}{L}_0)(G_0+\mathrm{j\&omega;}{C}_0),}$ $Z_c=\sqrt{\frac{R_0+\mathrm{j\&omega;}{L}_0}{G_0+\mathrm{j\&omega;}{C}_0}};$ The relationship between the voltage U _B1 at B1 and the voltage U B2 at _B2 is: U _B1 =U _B2 cos αl. Suppress the voltage increase at the end of the no-load line, that is, maintain the effective value of the bus voltage U _B2 at 1p.u. (pu is the highest operating phase voltage amplitude (standard value), 1p.u is 1 times the phase voltage.), That is |U _B1 |=cos αl. VSC-HVDC adopts the control method of constant DC voltage on the rectifier side and constant AC bus voltage on the inverter side. The block diagrams of the control system are shown in Figure 3 and Figure 4. Wherein, K ₁ =-2Q/3U, U _B1ref is the reference value of the AC bus voltage at B1, and U _B1 is the measured value of the AC bus voltage at B1. When U _B1ref is selected as 1, it is normal start, that is, the AC bus voltage at B1 reaches 1p.u, which is equivalent to the ideal voltage source running with no-load line; when U _B1ref is selected as soft start, that is to maintain |U _B1 |=cosαl, take This strategy suppresses power frequency overvoltage at the end of the no-load line.

In the present invention, the control structure of the rectification side is shown in Fig. 3, the PWM trigger module and the rest of the phase-locked loop in Fig. The physical quantities involved are parameters in the dq0 coordinate system obtained through Park transformation. The meaning of each parameter is as follows.

U _dref , U _dc are the reference value and measurement value of DC bus voltage respectively; i _1dref , i _1d , i _1qref , i _1q are the d-axis components of the rectification side AC current reference value and measurement value respectively; i _1qref , i _1q are respectively The q-axis components of the reference value and measured value of the AC current on the rectification side; u _s1d , u _s1q are the d-axis and q-axis components of the rectification side system AC voltage; u _c1d , u _c1q are the d-axis and q-axis components of the rectifier output voltage q-axis component; the functional relationship of unit 1 is: $m=\frac{2\sqrt{{\mathrm{<figure-callout\; id="2"\; label="u\; cd"\; filenames="S2007101854536E00021.png"\; state="\{\{state\}\}">u</figure-callout>}}_{\mathrm{cd}}^{}+u_{\mathrm{cq}}^2}}{u_d},$ The functional relationship of unit 2 is: $\mathrm{\&delta;}=\arctan \frac{u_{\mathrm{cd}}}{u_{\mathrm{cq}}};$ Q _1ref is the reactive power reference value output by the AC system on the rectification side.

The VSC-HVDC connected to the network is simulated to start the long-term unloaded line under the two starting modes of direct start and soft start. The lengths of the simulated lines are 250km and 500km respectively. Considering that the line is lossless and the line parameters have nothing to do with frequency , when the line length is 250km, the theoretical calculation uses VSC to directly start the no-load long line, and the power frequency overvoltage at the end of the line is 1.04p.u. When the line length is 500km, the theoretical calculation VSC directly starts the no-load long line. The power frequency overvoltage at the line end is 1.15p.u . Simulation results are shown in Figures 6 and 7. When the VSC adopts the direct start mode, the power frequency overvoltage is generated at the end of the no-load line (Figure 6); when the VSC adopts the flexible start mode, the power frequency overvoltage is significantly reduced (Figure 7), and can basically be maintained at 1p.u .

The control strategy of the present invention to suppress the operating overvoltage at the end of the closing no-load line is to control the effective value of the AC bus voltage at B1 to rise from 0.5p.u to 1p.u within one power frequency cycle. At this time, the control system on the rectification side is the same as in Figure 3. The inverter side control system is shown in Figure 5. The no-load line is closed at 1.5s, and the VSC is directly started and the VSC is started using the soft start of the control system shown in Figure 5. The overvoltage coefficient K of the phase A at the end of the line is the overvoltage amplitude and the steady-state voltage amplitude. Value ratio (the steady-state voltage is the power frequency overvoltage), as shown in Figures 8 and 9.

By comparing the operating overvoltage at the end of the line under the start-up mode in Figures 8 and 9, it can be obtained that when the length of the line is 250km, the power frequency overvoltage at the end is 1.04p.u. On this basis, the operating overvoltage generated by superposition is not obvious (Figure 8). However, after VSC flexible control, the terminal operating overvoltage gradually rises to the steady-state voltage (Figure 9), without impact; when the line length increases, the operating overvoltage at the moment of closing increases significantly (Figure 10), when VSC-HVDC adopts After soft start, as shown in Figure 11, the operating overvoltage is significantly reduced. It is proved that reducing the power frequency overvoltage at the initial stage of start-up can effectively suppress the operating overvoltage, and it is effective to adopt this control strategy to suppress the operating overvoltage.

Claims (3)

1, a kind of start-up method of no-load long line, it is characterized in that, it is that the inverter side of the networked VSC-HVDC is directly connected with the no-load long line (XL) to be started by circuit breaker (BRK), when starting Close the circuit breaker (BRK) first, and then make the AC side of the VSC-HVDC inverter electrified, and the inverter controls the line voltage, so that the effective value of the voltage recovers according to the specified curve. 2. The method for starting a long line with no load according to claim 1, wherein the inverter controls the line voltage to recover in the following manner: within 1 to 5 power frequency cycles from the start of the long line, the circuit breaks The effective value of the AC bus voltage at the BRK access point of the switch rises uniformly from 0.3 to 0.6p.u to 0.8 to 1p.u, and ensures that the voltage at the end of the no-load line does not exceed 1.1p.u. 3. The method for starting a long no-load line according to claim 1, wherein the modulation degree (m) of the inverter is determined by the following formula: $\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cq</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}}<span\; class="notranslate">\; ,</span>$ The phase angle difference between the fundamental component of the converter output voltage (U _c ) and the fundamental component of the AC bus voltage (U _s ) is determined by the following formula: $\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; cq</span>}}}<span\; class="notranslate">\; ;</span>$ Among them, u _cd , u _cq , are the AC side voltages in the dq0 coordinate system; U _d is the DC voltage.

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