CN110518617B - MMC-MTDC droop control method considering operation state of alternating current side - Google Patents

Abstract

An MMC-MTDC droop control method considering an alternating current side operation state is characterized in that aiming at a multi-terminal flexible direct current transmission system, an improved droop control strategy is adopted to control a voltage source converter station in the system, a voltage droop coefficient is adjusted in real time according to the frequency and load rate change of the alternating current system, and the specific expression is as follows:

wherein

The control mode of the voltage source converter station is adjusted according to the change conditions of the frequency and the load factor of the alternating current system, so that the system can always run under a proper working condition, the direct current voltage deviation of a line can be reduced, the alternating current system can be prevented from being overloaded or the frequency can be prevented from exceeding an allowable range, and the stability of the interconnected system is improved.

Description

MMC-MTDC Droop Control Method Considering the Operating State of AC Side

technical field

The invention relates to a power coordination control method of a direct current transmission system considering the operating state of the alternating current side, and belongs to the technical field of power transmission and distribution.

Background technique

Compared with traditional AC transmission, HVDC transmission has obvious advantages in long-distance power supply, interconnection of systems with different frequencies, and long submarine cable transmission. Compared with two-terminal HVDC transmission, multi-terminal DC transmission (MTDC) can realize power supply from multiple power sources, power reception from multiple drop points, and more flexible, cost-effective power flow control. Modular multilevel converter (MMC) has the advantages of flexible structure, convenient expansion, high voltage and high power energy conversion, easy formation of multi-terminal system, and power supply to passive grid. Therefore, the MMC-MTDC power transmission system will become an important direction for the development of the multi-terminal direct current transmission system in the future.

The control objective of the multi-terminal DC transmission system is to realize the reasonable distribution of DC side power and the stability of DC bus voltage. The various coordinated control strategies currently proposed mainly consider the use of relevant information on the DC side to realize the coordinated distribution of power among the converter stations within the DC side, while less consideration is given to the coordinated distribution of AC system operating status to participate in the coordinated distribution of DC side power. In the actual power grid, the unbalanced power allocated to each interconnected grid should not only consider the power margin of the converter station, but also consider the operating status of the AC side grid, such as the frequency and load rate changes of the AC system, and the spinning reserve capacity of some grids Even if the power margin of the converter station connected to it is large, it cannot share more unbalanced power after the unbalanced power of the DC grid occurs, otherwise the AC grid will experience excessive frequency fluctuations. In the same way, if the load rate of the AC system is not considered, it is easy to cause the individual AC system to be fully loaded and thus lose the ability to regulate the entire system.

At present, there are mainly three types of control methods for HVDC transmission: master-slave control, DC voltage margin control and voltage droop control. However, the current control strategy mainly has the following shortcomings:

(1) The master-slave control has high requirements on the communication system, and the system stability will face a great threat when the communication is interrupted, so this seriously restricts the development of this control method.

(2) Voltage margin control is prone to power oscillation during the switching process of the control mode of the converter station, the selection of the voltage margin is more complicated, and the priorities of several DC voltage converter stations are difficult to interact with each other when the scale of the converter station is large Cooperate.

(3) When the converter station adopts the traditional fixed-slope droop control, the droop coefficient cannot be flexibly adjusted according to the operating status of the AC side, which will easily lead to the full load of the AC system and the frequency exceeds the allowable range, thereby reducing the stability of the interconnected system.

To sum up, the existing control methods of HVDC transmission are not ideal, and it is necessary to improve them.

Contents of the invention

The purpose of the present invention is to address the drawbacks of the prior art and provide an MMC-MTDC droop control method considering the operating state of the AC side to prevent the AC system from being fully loaded and the frequency from exceeding the allowable range and improve the stability of the interconnected system.

Problem described in the present invention is solved with following technical scheme:

An MMC-MTDC droop control method considering the operating state of the AC side. The method is aimed at a multi-terminal flexible direct current transmission system, and uses an improved droop control strategy to control the voltage source converter station in the system. The voltage droop coefficient is based on the frequency of the AC system and the change of load rate to adjust in real time, the specific expression is:

in

In the formula: K _i is the voltage droop coefficient of converter station i; K _i 0 is the initial voltage droop coefficient set by converter station i according to its rated capacity; Δf _thre is the frequency threshold; η is the AC system load rate; η' is the setting value of the load rate of the AC system; f is the measured value of the frequency of the AC system; f ₀ is the initial value of the frequency of the AC system; Δf' is the setting value of the frequency fluctuation of the AC system.

In the above MMC-MTDC droop control method considering the running state of the AC side, the AC system frequency fluctuation set value Δf' is set to 0.2Hz; the AC system load rate set value η' is set to 0.85; the frequency threshold Δf _thre is set to 0.02 Hz.

The invention adjusts the control mode of the voltage source converter station according to the change of the frequency of the AC system and the load rate of the AC system, so that the system can always operate under a suitable working condition, which can not only reduce the DC voltage deviation of the line, but also prevent the overload of the AC system Or the frequency is outside the allowed range, which increases the stability of the interconnected system.

Description of drawings

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

Figure 1 is a structural diagram of a three-terminal flexible DC transmission system;

Fig. 2 is a comparison chart of U _dc -P _dc characteristic curves of traditional droop control and improved droop control;

Figure 3 is the DC voltage simulation waveform after a small unbalanced power appears on the DC side;

Figure 4 is the simulation waveform of the output power of the AC side after a large unbalanced power appears on the DC side;

The symbols used in the drawings or in the text are: HVDC is high-voltage direct current transmission, MMC-MTDC is multi-terminal flexible direct current transmission, K _i is the voltage droop coefficient of converter station i, Δf _thre is the frequency threshold, and Δf' is the frequency fluctuation setting of the AC system η is the load rate of the AC system, η' is the set value of the load rate of the AC system, P _dc is the measured value of the active power of the DC side, f is the measured value of the frequency, f ₀ is the initial value of the frequency, and U _dc is the actual value of the DC voltage , P is the actual measured value of active power from the AC system, P _N is the rated active power value of the AC system, and K _i ⁰ is the initial voltage droop coefficient set by converter station i according to its rated capacity.

Detailed ways

The invention provides an MMC-MTDC droop control method considering the running state of the AC side, so as to improve the voltage quality and the stability of the interconnection system.

The present invention is aimed at the voltage source converter station in the multi-terminal flexible direct current transmission system, adopts the improved droop control strategy to control, and the expression of the voltage droop coefficient is:

in

In the formula: K _i ⁰ is the initial voltage droop coefficient set by converter station i according to its rated capacity; Δf _thre is the frequency threshold; η is the load rate of the AC system; η' is the set value of the load rate of the AC system; System frequency measurement value; f ₀ is the initial value of AC system frequency; Δf' is the setting value of AC system frequency fluctuation.

Considering that the frequency fluctuation range of long-term stable operation of the AC system does not exceed [-0.2, 0.2] Hz, in the present invention, Δf' is set to 0.2 Hz; the load rate setting value of the AC system η' is set to 0.85; Minor fluctuations in frequency lead to frequent changes in the slope coefficient value of the corresponding converter station. In the improved droop control, a frequency threshold is set for the frequency change. The frequency threshold set by the present invention is 0.02Hz. Only when the frequency is lower than the lower limit of 49.98Hz or exceeds The droop coefficient will only change when the upper limit is 50.02Hz.

According to the different operating states of the AC system, after adopting this improved droop control strategy, the converter station will have the following three control modes:

(1) Control mode 1: Each AC side system operates stably, there is no power disturbance or power fluctuation in the DC network, the frequency fluctuation of the AC side is less than the frequency threshold, and the converter station operates normally according to a fixed slope.

(2) Control mode 2: When there is a small unbalanced power on the DC side, such as a small accident in an AC system, when other AC systems participate in the adjustment of unbalanced power on the DC side, their own frequency fluctuations exceed the frequency threshold, but these When the load rate and frequency variation of the AC system do not exceed their respective set values, the droop coefficient of each converter station is reduced on the basis of the initial voltage droop coefficient. At this time, the slope of the voltage droop curve becomes smaller, the voltage rigidity is good, and the system runs In the state that focuses on stabilizing the DC voltage, the system operating DC voltage is closer to the reference operating voltage during power regulation.

(3) Control mode 3: When there is a large unbalanced power on the DC side, such as when a certain AC system has a major accident or a certain converter station is out of operation, when other AC systems participate in the unbalanced power regulation on the DC side, the When the frequency fluctuation exceeds the frequency threshold, and at least one of the AC system load rate and frequency change exceeds its respective set value, the droop coefficient of each converter station will increase on the basis of the initial voltage droop coefficient, and the slope of the droop curve will become larger. The apportioned power is reduced, and the system operates in a state that focuses on power allocation, preventing the AC system connected to it from distributing too much power, thereby effectively preventing the AC system from being overloaded or the frequency exceeding the allowable range.

The method will be described in detail below in conjunction with the embodiments, referring to Figure 1, the three-terminal flexible direct current transmission system includes:

The AC system 1 is connected to the converter station 2;

The converter station 2 is connected with the AC system 1 and the bus bar 3. The converter station operates in the rectification state, and is used to convert the AC power of the AC system 1 into DC power and transmit it to the DC bus bar 3;

The DC bus 3 is connected to the converter station 2, the converter station 9 and the DC transmission line 4, and the bus is used to gather the electric energy transmitted by the converter station 2 and the converter station 9, and then transmit it through the DC transmission line 4;

The direct current transmission line 4 is connected to the bus bar 3 and the bus bar 5, and the direct current transmission line is used for high-voltage direct current transmission;

The AC system 7 is connected with the converter station 6 . The busbar 5 is connected to the DC transmission line 4 and the converter station 6, and the busbar is used to gather the electric energy of the DC transmission line 4, and then supplies power to the AC system 7 through the converter station 6;

The converter station 6 is connected to the bus 5 and the AC system 7. The converter station operates in an inverter state and is used to convert the DC power of the DC bus 5 into AC power and transmit it to the AC system 7;

The AC system 8 is connected to the converter station 9;

The converter station 9 is connected to the AC system 8 and the bus bar 3. The converter station operates in a rectification state and is used to convert the AC power of the AC system 8 into DC power and transmit it to the DC bus bar 3;

The converter station 2 of the present invention adopts constant active power control, and the converter station 6 and the converter station 9 both adopt droop control. The specific steps of the MMC-MTDC improved droop control strategy considering the operating state of the AC side are as follows:

Step 1: Detect the frequency variation Δf of the AC system and the load rate η of the AC system, the expression of which is: Δf=|ff ₀ |, η=P/P _N . Where f is the measured value of the AC system frequency; f ₀ is the initial value of the AC system frequency; P is the measured value of the active power emitted by the AC system; P _N is the rated active power value of the AC system.

正常运行；当Δf_thre＞0.02Hz时换流站切换到改进下垂系数K′_i运行。The second step: judge the control mode of the converter station according to the threshold value Δf _thre of the frequency variation. When Δf _thre ≤0.02Hz, the converter station follows the fixed droop coefficient

Normal operation; when Δf _thre >0.02Hz, the converter station switches to the improved droop coefficient K′ _i operation.

Step 3: Calculate the ratio of the AC side frequency variation Δf to the frequency fluctuation set value Δf' and the AC system load rate η to the load rate set value η' respectively. Among them, Δf' is set as 0.2Hz, and η' is set as 0.85.

两个量的大小判断换流站在改进下垂控制下的控制模式。当

时，改进下垂系数根据交流侧频率变化和负载率变化而减小，其表达式为：

此时电压下垂曲线斜率变小，电压刚性好，系统运行在侧重于稳定直流电压的状态；当

或

时，自适应下垂系数根据交流侧频率变化和负载率变化而增大，其表达式为：

此时下垂曲线斜率变大，换流站分配的功率减小，系统运行在侧重于功率分配的状态，防止其所连接的交流系统分配的功率过多，从而有效避免交流系统过载或者频率超出允许的范围。Step 4: According to

The magnitude of the two quantities determines the control mode of the converter station under the improved droop control. when

When , the improved droop coefficient decreases according to the change of AC side frequency and load rate, and its expression is:

At this time, the slope of the voltage droop curve becomes smaller, the voltage rigidity is good, and the system operates in a state that focuses on stabilizing the DC voltage; when

or

When , the adaptive droop coefficient increases according to the change of AC side frequency and load rate, and its expression is:

At this time, the slope of the drooping curve becomes larger, the power distributed by the converter station decreases, and the system operates in a state that focuses on power distribution, preventing the AC system connected to it from distributing too much power, thereby effectively preventing the AC system from being overloaded or the frequency exceeding the allowable range.

The improved droop control strategy proposed by the present invention can adjust the working state of the DC side converter station to focus on stabilizing the DC voltage or focusing on power distribution according to the change of the frequency of the AC system and the change of the load rate of the AC system, by adjusting the system to run in a suitable working condition , reducing the voltage deviation of the DC side, preventing the AC system from being fully loaded and the frequency exceeding the allowable range, and improving the operation stability of the interconnected system.

See Figure 2. In traditional droop control, the droop coefficient is a fixed value. When there is a power shortage in the DC system, the power shared by each converter station is a fixed value. This will easily lead to large frequency fluctuations or AC The system load rate is too high, affecting system stability.

In the improved droop control strategy proposed by the present invention, the voltage droop coefficient expression is:

in

In the improved droop control, the voltage droop coefficient is not a fixed value, but is automatically adjusted according to the load rate of the AC system and the variation of the frequency of the AC system.

改进下垂系数根据交流侧频率变化和负载率变化而减小，其表达式为：

此时电压下垂曲线斜率变小，电压刚性好，系统运行在侧重于稳定直流电压的状态；在CD和EF段，此时

改进下垂系数根据交流侧频率变化和负载率变化而增大，其表达式为：

此时下垂曲线斜率变大，换流站分摊的功率减小，系统运行在侧重于功率分配的状态，从而有效避免其所连接的交流系统分配过多的功率，防止交流系统过载或者频率超出允许的范围。In Fig. 2, the solid line represents the traditional droop control strategy, because the droop coefficient is a fixed value, which is represented as a slanted straight line in the figure. The dotted line represents the improved droop control strategy, in the AB segment, at this time

The improved droop coefficient decreases according to the change of AC side frequency and load rate, and its expression is:

At this time, the slope of the voltage droop curve becomes smaller, the voltage rigidity is good, and the system operates in a state that focuses on stable DC voltage; in the CD and EF sections, at this time

The improved droop coefficient increases according to the change of AC side frequency and load rate, and its expression is:

At this time, the slope of the drooping curve becomes larger, the power shared by the converter station decreases, and the system operates in a state that focuses on power distribution, thereby effectively preventing the AC system connected to it from distributing too much power, preventing the AC system from being overloaded or the frequency exceeding the allowable range.

Figure 3 is the DC voltage simulation waveform after a small unbalanced power appears on the DC side

It can be seen from Fig. 3 that the unbalanced power appears on the DC side, and after the power regulation of each converter station, the voltage change of the converter station using the improved droop control is obviously smaller than that of the traditional droop control, because when When there is a small unbalanced power on the DC side, when other AC systems participate in the adjustment of unbalanced power on the DC side, their own frequency fluctuation exceeds the frequency threshold, but the load rate of the AC system and the frequency change of the AC side do not exceed their respective set values, The droop coefficient of each converter station is reduced on the basis of the initial voltage droop coefficient. At this time, the slope of the voltage droop curve becomes smaller, the voltage rigidity is good, and the system operates in a state that focuses on stabilizing the DC voltage. The voltage is closer to the reference operating voltage.

Figure 4 is the simulation waveform of the output power of the AC side after a large unbalanced power appears on the DC side

It can be seen from Figure 4 that after a large unbalanced power appears on the DC side, the output active power increase of the AC system connected to the converter station using improved droop control is significantly lower than that of the converter station connected to traditional droop control. The increase of the active power output by the AC system avoids the full load of the AC system. This is because when a large unbalanced power appears on the DC side, the frequency fluctuation exceeds the frequency threshold, and at least one of the load rate of the AC system and the frequency change of the AC side exceeds At this time, the droop coefficient of each converter station increases on the basis of the initial voltage droop coefficient, the slope of the droop curve becomes larger, the power shared by the converter station decreases, and the system operates in a state that focuses on power distribution, which can Effectively prevent the converter station from distributing too much power, and prevent the AC system from being overloaded or the frequency exceeding the allowable range.

Claims (2)

1. A MMC-MTDC droop control method considering the AC side operating state, it is characterized in that, the method is aimed at the multi-terminal flexible direct current transmission system, adopts the improved droop control strategy to control the voltage source converter station in the system, the voltage The droop coefficient is adjusted in real time according to the change of AC system frequency and load rate, and the specific expression is:

in

In the formula: K _i is the voltage droop coefficient of converter station i;

is the initial voltage droop coefficient set by converter station i according to its rated capacity; Δf _thre is the frequency threshold; η is the load rate of the AC system; η' is the set value of the load rate of the AC system; f is the measured value of the frequency of the AC system; ₀ is the initial value of AC system frequency; Δf' is the setting value of AC system frequency fluctuation. 2. A kind of MMC-MTDC droop control method considering AC side running state according to claim 1, is characterized in that, described AC system frequency fluctuation setting value Δf' is set to 0.2Hz; AC system load rate setting The value η' is set to 0.85; the frequency threshold Δf _thre is set to 0.02 Hz.

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