CN105071424B - A kind of high-voltage direct current control method - Google Patents

Abstract

The invention discloses a high-voltage direct current system control method, that is, VDDCFP is set after the VDCOL controller, and the VDDCFP control process is: S1: Real-time monitoring of the three-phase voltage amplitude on the inverter side, and obtaining the instantaneous value of the voltage through Fourier decomposition Each harmonic voltage value; S2: voltage distortion current limiting, calculate the harmonic distortion rate through the amplitude of each harmonic voltage after Fourier decomposition, and take the largest harmonic distortion rate in the three-phase voltage as the feedback signal; S3: DC current deviation current limiting, calculate the difference between the DC current instantaneous value I _d and the rated value I _{d_ref} in the DC current setting value, and this difference is used as the input signal of the gain process together with the feedback signal in step S2; S4: Gain, selected through an optimization algorithm, and select an appropriate gain size. In the present invention, when the harmonic distortion rate of the commutation bus voltage is high, the influence of the harmonics on the time area of the commutation voltage is insufficient to cause commutation failure by reducing the direct current.

Description

A high voltage direct current system control method

technical field

The invention relates to the field of high-voltage direct current system control, in particular to a control method for preventing commutation failure in a high-voltage direct current transmission system.

Background technique

Due to its strong controllability and large transmission capacity, DC transmission is widely used in long-distance power transmission and has high economic benefits. Commutation failure is a principle problem of traditional DC transmission system based on thyristor commutation principle. With the development of my country's UHV DC transmission technology, the power of single-circuit DC transmission is getting larger and larger, and the number of multi-feed DC drop points is increasing. When commutation failure occurs, the disturbance to the connected AC system is also increasing. The larger the value, the more consecutive commutation failures will cause the DC transmission system to be blocked.

Voltage drop and voltage distortion are the main causes of commutation failure, and voltage distortion can be expressed as a combination of harmonics through Fourier decomposition. At present, most of the research is based on the commutation failure prevention measures (such as CFPREV, commutation failure prevention) and recovery strategies based on the fundamental voltage reduction. The traditional low-voltage current limiting unit VDCOL (Voltage Dependent Current Order Limiter) control is to detect When the DC voltage decreases, the commutation failure is avoided by limiting the DC current, without considering the influence of harmonic distortion. In fact, with the increase in the complexity of the power grid and the use of a large number of power electronic devices, there are more and more factors that cause voltage distortion of the DC converter bus, and in the recovery stage of the DC transmission system, the rise of the DC current will cause the saturation of the converter transformer , the generation of a large number of harmonics will also cause voltage distortion, which will cause commutation failure. At present, there are few researches on the control methods to suppress the commutation failure caused by harmonics.

Contents of the invention

The technical problem to be solved by the present invention is to provide a high-voltage DC transmission commutation failure prevention and control method. When the harmonic distortion rate of the commutation bus voltage is high, by reducing the DC current, the harmonics can reduce the time area of the commutation voltage. The impact is not strong enough to cause commutation failure.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A high-voltage DC system control method, a commutation failure current limiting prevention controller VDDCFP (Voltage Distortion Dependent Commutation Failure Prevention) is set after the VDCOL controller, and the VDDCFP control process is as follows:

S1: Monitor the three-phase voltage amplitude on the inverter side in real time, and obtain the harmonic voltage values of the instantaneous voltage value through Fourier decomposition. The Fourier decomposition expression is: In the formula, u(t) is the instantaneous voltage value, ω _n is the nth angular frequency, and F(ω _n ) is the nth harmonic voltage amplitude;

S2: Voltage distortion current limiting. The harmonic distortion rate is calculated by the amplitude of each harmonic voltage after Fourier decomposition, and the maximum harmonic distortion rate in the three-phase voltage is taken as the feedback signal. The formula for calculating the harmonic distortion rate is: : _In the formula, E1 is the fundamental voltage amplitude;

S3: DC current deviation current limiting, calculate the difference between the DC current instantaneous value I _d and the rated value I _{d_ref} in the DC current setting value, and this difference is used as the input signal of the gain process together with the feedback signal in step S2;

S4: Gain, selected through an optimization algorithm, and select an appropriate gain size.

The control method of the present invention also includes S5: setting the start-up control process after the voltage distortion current limit and the DC current deviation current limit, and determining the threshold a, when a is greater than the feedback signal b, the VDDCFP controller does not work, and when a is not greater than the feedback signal b When signal b, the VDDCFP controller works.

The control method of the present invention further includes limiting the amplitude of the current I _out after the gain process.

In step S4 of the control method of the present invention, the optimization algorithm of the gain process is the simplex method or the dichotomy method.

Compared with the prior art, the beneficial effects of the present invention are: real-time monitoring of the three-phase voltage amplitude on the inverter side, calculating the harmonic distortion rate through Fourier decomposition, and taking the maximum harmonic distortion rate as the feedback signal, and finally through The gain link reduces the setting value of the DC current, so that the influence of harmonics on the time area of the commutation voltage is not enough to cause commutation failure. At the same time, the output current is limited, which can effectively prevent the serious deviation of the current setting value during the transient process with a high harmonic distortion rate. Because the harmonic content is very little when the system is running in a steady state, the output of the VDDCFP controller is also small, which does not affect the normal operation of the DC transmission system.

Description of drawings

Figure 1 is a schematic diagram of the structure of a VDDCFP controller added to a HVDC power transmission system.

Figure 2 shows the structure and parameters of the CIGRE DC transmission first standard test system.

Fig. 3 is a structural diagram of the CIGRE DC transmission first standard test system after the VDDCFP controller is added in the present invention.

Fig. 4 is a response diagram of the arc extinguishing angle after third harmonic injection using the method of the present invention.

Fig. 5 is a DC current response diagram after third harmonic injection using the method of the present invention.

Fig. 6 is a response diagram of arc extinguishing angle for a three-phase short-circuit fault using the method of the present invention.

Fig. 7 is a diagram of a DC current response to a three-phase short-circuit fault using the method of the present invention.

In the figure: MAX means that the maximum harmonic distortion rate in the three-phase voltage is taken as the feedback signal; 0.0 means that the VDDCFP controller does not work; K is the gain process; U _a , U _b , and U _c are the instantaneous values of the three-phase voltage; U _DC is the DC voltage of the inverter side; Rv is a compound resistance, which is used to determine the starting voltage of VDCOL is determined by the DC voltage of which point on the DC line; Is the filtering link; VDCOL has four key parameters, namely the maximum DC current I _H , the minimum DC current I _L , the high voltage threshold U _H and the low voltage threshold U _L ; I _out is the DC current setting value.

detailed description

The purpose of the present invention is to provide a high-voltage DC system control method that can suppress commutation failure caused by harmonics, mainly adding a commutation failure current-limiting prevention controller that considers harmonic distortion rate after the VDCOL on the inverter side of the DC transmission system VDDCFP. The principle is that the commutation voltage time area required for the commutation process of the DC transmission system is fixed (determined by the DC current _Id ), harmonics will cause voltage distortion, and voltage distortion will increase the corresponding commutation time, thereby increasing the The commutation overlap angle reduces the arc extinguishing angle and causes commutation failure. Therefore, when the harmonic distortion rate of the commutation bus voltage is high, the influence of the harmonics on the time area of the commutation voltage is insufficient by reducing the DC current resulting in commutation failure.

When VDDCFP is running normally, the DC transmission system detects that the voltage U _DC on the inverter side drops, and performs current limiting. The current limiting value operates according to the VDCOL setting value. Different DC voltage drops correspond to different operating currents, and then the current limiting The DC current I _out is output to the DC side, thereby changing (generally reducing) the DC current value of the entire system and preventing commutation failure from occurring.

Control method of the present invention is realized by the following steps:

1) Monitor the three-phase voltage amplitude on the inverter side in real time, and obtain the harmonic voltage values of the instantaneous voltage values through Fourier decomposition. The Fourier decomposition expression is shown in (1) formula.

In the formula, u(t) is the instantaneous voltage value, ω _n is the nth angular frequency, and F(ω _n ) is the nth harmonic voltage amplitude.

2) Calculate the harmonic distortion rate through the amplitude of each harmonic voltage after Fourier decomposition, and take the largest harmonic distortion rate in the three-phase voltage as the feedback signal. The calculation formula of harmonic distortion rate is shown in formula (2).

_In the formula, E1 is the fundamental voltage amplitude.

3) In the event of a fault, the decrease in the voltage on the inverter side will cause a sudden increase in the DC current, resulting in an increase in the voltage time area required for the commutation process, further increasing the possibility of commutation failure. Therefore, the DC current difference link is added after the current limit, and the difference between the DC current instantaneous value and the rated value is considered in the DC current setting value. This difference is used together with the feedback signal as the input signal to the gain process.

4) The start-up link is added after the voltage distortion current limiting link and the DC current deviation current limiting link, and the controller is prevented from malfunctioning by setting the threshold, so that it does not affect the steady-state operation of the DC transmission system. Determine the threshold a, when a is greater than the feedback signal b, the VDDCFP controller does not work, and when a is not greater than the feedback signal b, the VDDCFP controller works.

5) Finally, set the gain link. The size of the gain reflects the influence of the harmonic distortion rate on the DC current reference value. If the gain is too large, the system will be too sensitive to the harmonic distortion rate and reduce the transmission power of the DC transmission system. If the gain is too small, the commutation failure will be increased. Odds of happening. The size of the gain can be selected by an optimization algorithm, such as the simplex method, the dichotomy method, and the like. Set the limiting link after the gain link to prevent the DC current from being too low and affecting the power transmission.

The International Council on Large Electric systems (CIGRE) DC transmission first standard test system is selected as a simulation example. The system structure and parameters are shown in Figure 2. The complete structure of the CIGRE model controller after adding VDDCFP is shown in Figure 3.

The VDDCFP control parameters are set according to the above steps. When the CIGRE model is running normally, the transmission current is 2kA, so its DC reference value is I _d-ref = 2, and the threshold is set to 0.05, that is, when the harmonic distortion rate exceeds 5%, the controller VDDCFP action. The gain value is set to 2 by bisection optimization. The maximum and minimum values of the limiting link are 1.2 and 0.55, respectively.

Verify the accuracy of the control method of the present invention by digital simulation software PSCAD, set following fault:

Fault 1: A third harmonic current with an amplitude of 0.5kA is injected into the commutation bus on the inverter side in 2s, and the fault lasts for 0.1s.

Fault 2: A three-phase ground short-circuit fault is set at the commutation bus on the inverter side at 2s, the fault lasts for 0.1s, and the grounding impedance is 0.01Ω.

Figure 4 and Figure 5 show the system response diagrams under fault-1 working condition. When there is no VDDCFP controller, commutation failure occurs after harmonic injection, and the DC current fluctuates greatly, which causes strong oscillation in the receiving end system. After adding the VDDCFP controller, the arc extinguishing angle increases to more than 7 degrees, no commutation failure occurs, and the DC current fluctuation is also small.

The system response under the second fault condition is shown in Figure 6 and Figure 7. Although there is no harmonic injection in the short-circuit fault, after the fault is removed, the rise of the DC current during the restoration of the DC transmission system will cause the saturation of the converter transformer and generate a large number of harmonics As a result, commutation fails. As shown in Figure 6 without the controller waveform, after the fault is removed, the system has a second phase commutation failure. After adding the controller, the system can return to the normal operation state more smoothly after the short-circuit fault, and there is no subsequent commutation failure.

Simulation results show that after adding VDDCFP control, the system can recover more smoothly after a fault, reducing the oscillation of system voltage and power. The control method of the present invention can effectively prevent commutation failure caused by harmonic injection in DC transmission system.

Claims (4)

1. a high-voltage direct current system control method is characterized in that, commutation failure current limiting prevention controller VDDCFP is set after the VDCOL controller, and the VDDCFP control process is as follows: S1: Monitor the three-phase voltage amplitude on the inverter side in real time, and obtain the harmonic voltage values of the instantaneous voltage value through Fourier decomposition. The Fourier decomposition expression is: In the formula, u(t) is the instantaneous voltage value, ω _n is the nth angular frequency, and F(ω _n ) is the nth harmonic voltage amplitude; S2: Voltage distortion current limiting. The harmonic distortion rate is calculated by the amplitude of each harmonic voltage after Fourier decomposition, and the maximum harmonic distortion rate in the three-phase voltage is taken as the feedback signal. The formula for calculating the harmonic distortion rate is: : _In the formula, E1 is the fundamental voltage amplitude; S3: DC current deviation current limiting, calculate the difference between the DC current instantaneous value I _d and the rated value I _{d_ref} in the DC current setting value, and this difference is used as the input signal of the gain process together with the feedback signal in step S2; S4: Gain, selected through an optimization algorithm, and select an appropriate gain size. 2. The high-voltage DC system control method according to claim 1, further comprising S5: setting the start-up control process after the voltage distortion current limit and the DC current deviation current limit, and determining the threshold a, when a is greater than the feedback signal b When , the VDDCFP controller does not work, when a is not greater than the feedback signal b, the VDDCFP controller works. 3. The high voltage direct current system control method as claimed in claim 1 or 2, characterized in that, after the gain process, the amplitude of the current I _out is limited. 4. The high voltage direct current system control method according to claim 1 or 2, characterized in that, in step S4, the optimization algorithm of the gain process is the simplex method or the dichotomy method.