Active frequency droop control method for AC system based on line power flow controller

Abstract

The present invention discloses a method for controlling the active frequency droop of an AC system based on a line-to-line flow controller. First, the converter connected to the auxiliary control line of the line-to-line flow controller is unlocked, and the line-to-line flow controller connected to the auxiliary control line is set to operate in a constant DC voltage and constant reactive power control mode; then, the converter connected to each main control line of the line-to-line flow controller is unlocked in sequence, and the line-to-line flow controller connected to each main control line is set to operate in a constant AC line active power and constant AC line reactive power control mode; then, the AC system frequency difference signal Δ is extracted at the receiving end of the main control line. f ac ; and extract the frequency difference signal Δ f ac Multiply by the droop coefficient K An active frequency droop signal is generated; finally, the active frequency droop signal is added to the active power control instructions of the inter-line power flow controllers connected to each master line, implementing droop control based on the inter-line power flow controllers. This invention can accurately control the active power of a specified line and support the frequency stability of the AC system.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current transmission, and in particular to an active frequency droop control method of an alternating current system based on a line power flow controller. Background Art

[0002] To effectively control AC power flows within AC power grids, utilizing Flexible AC Transmission Systems (FACTS) to address the problem of blockage at critical sections caused by uneven power flow distribution has become a top priority. Compared to conventional AC equipment, FACTS devices based on power electronics offer advantages such as rapid response, continuous adjustment, and flexible control, significantly improving the transmission capacity and efficiency of power systems. FACTS technologies, such as the Unified Power Flow Controller (UPFC), have been put into operation in the Jiangsu power grid, making significant contributions to power flow control and voltage stability. However, the UPFC can only control power flows on specific lines, and controlling power flows on its own lines can potentially cause power flows on nearby heavily loaded lines to exceed their limits. During the 14th Five-Year Plan period, during the winter peak period of renewable energy generation, such as wind and photovoltaic power, heavy power flows are expected to occur on multiple cross-river channels under the "North-to-South Power Transmission" strategy, with N-1 and N-2 power flow overloads. At this time, relying solely on the power flow control capabilities of the UPFC will no longer be enough to address the power flow imbalances and overloads in the Jiangsu power grid.

[0003] At the same time, the Interline Power Flow Controller (IPFC), a representative device of the third generation FACTS, has shown greater potential with its excellent performance. Figure 1 As shown, the simplified IPFC structure diagram is as follows Figure 2As shown. On the one hand, IPFC not only directly controls the power flow on the transmission lines where it is installed in series, like UPFC, but also enables power exchange between lines, thereby controlling the power flow between different lines. On the other hand, while UPFC's power flow control on its own installed line may cause the power flow on nearby heavily loaded lines to exceed the limit, IPFC's power flow control is directional, "transferring" the power flow from heavily loaded lines to nearby lightly loaded lines, reducing the impact on the power flow on other lines. Overall, IPFC can dynamically control the active power, reactive power, voltage, impedance, and power angle of multiple lines, facilitating system operation optimization and improving transient stability, and has broad application prospects. Furthermore, IPFC can, through its own topology switching, continue to control the system power flow by utilizing the converters on the remaining lines after a controlled line fails, preventing further deterioration of the situation after the failure, thus providing high reliability. Therefore, IPFC has broad application prospects for resolving transmission bottlenecks caused by uneven power flow distribution in existing load-intensive power grids and improving the flexibility of power grid control.

[0004] Similar to the principles and characteristics of UPFC, the active power exchange between the converters in an IPFC is in a state of dynamic equilibrium. This means that the IPFC equipment itself neither absorbs nor generates active power relative to the entire system. During normal IPFC operation, a specific line is selected as its auxiliary control line, while the remaining lines serve as the main control lines. The converters in the main control line control both the active and reactive power of the line in which it resides. Since the converters in the auxiliary control lines need to maintain DC bus voltage stability, they only control the active or reactive power of the line in which they reside. This control method allows one or several heavily loaded lines to be selected as the main control lines. While ensuring that the power flow on the main control line does not exceed the limit, the overloaded power flow is rationally distributed to other less-priority lines. This effectively balances the power flow across transmission channels, prevents power flow overload, and significantly improves the system's transmission capacity.

[0005] The DC voltage droop control strategy is mainly used in multi-terminal flexible DC (VSC-MTDC) systems. The control concept of the DC voltage droop control strategy is derived from the frequency modulation controller in the AC system. There are two modes: DC voltage-DC power droop control and DC voltage-DC current droop control. The working principles of the two modes are the same. This section takes DC voltage-DC power droop control as an example. Its control characteristic diagram and controller structure are shown in Figure 2. Figure 3 As shown in (a) and (b).

[0006] Existing measures:

[0007] (1) Measure 1: Generator primary frequency regulation, you can refer to the following references:

[0008] [Reference 1] Yu Daren, Guo Yufeng. Online estimation of primary frequency regulation capability of power grid[J]. Proceedings of the CSEE, 2004(03):77-81.

[0009] However, this measure can only adjust the generator's extreme output power through one-time frequency regulation, and cannot adjust the power for a specified AC line.

[0010] (2) Measure 2: Flexible DC grid droop control, refer to the following references:

[0011] [Reference 2] Zhu Shanshan, Wang Fei, Guo Hui, Wang Qifeng, Gao Yanxia. A review of research on droop control technology for DC microgrids[J]. Proceedings of the CSEE, 2018, 38(01):72-84+344. DOI:10.13334 / j.0258-8013.pcsee.171408.

[0012] However, the DC grid droop control of this measure is mainly used to adjust the matching of DC side power and DC voltage, and does not perform droop adjustment on AC power. Summary of the Invention

[0013] To address the above issues, the present invention aims to provide an AC system active frequency droop control method based on a line power flow controller, which can accurately control the active power of a specified line and support AC system frequency stability. The technical solution is as follows:

[0014] A method for controlling active frequency droop of an AC system based on a line power flow controller comprises the following steps:

[0015] Step 1: Unlock the converter connected to the auxiliary control line of the line power flow controller;

[0016] Step 2: Set the power flow controller between the auxiliary control lines to operate in constant DC voltage and constant reactive power control mode;

[0017] Step 3: Unlock the converters connected to each main control line of the line power flow controller in turn;

[0018] Step 4: Set the power flow controllers between the lines connected to each master control line to operate in the fixed AC line active power and fixed AC line reactive power control modes in sequence;

[0019] Step 5: Extract the AC system frequency difference signal Δf at the receiving end of the main control line ac ;

[0020] Step 6: Extract the frequency difference signal Δf ac Multiply by the droop coefficient K to obtain the active frequency droop signal;

[0021] Step 7: Add the active frequency droop signal to the active power control instruction of the line power flow controller connected to each master line to implement droop control based on the line power flow controller.

[0022] The beneficial effects of the present invention are as follows: existing IPFC research does not involve the design of a droop control strategy. Compared with the primary frequency regulation of the generator, IPFC can accurately control the active power of the specified line. Compared with DC transmission, the IPFC droop strategy can support the frequency stability of the AC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the IPFC structure.

[0024] Figure 2 A simplified diagram of the IPFC structure

[0025] Figure 3 DC voltage droop controller; (a) control characteristics; (b) controller structure

[0026] Figure 4 This is the test system topology diagram.

[0027] Figure 5 This is the IPFC droop control strategy diagram.

[0028] Figure 6 The comparison diagram of the receiving end AC system frequency with and without the droop control strategy.

[0029] Figure 7 A partially enlarged view comparing the AC system frequency at the receiving end with and without the droop control strategy. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] A method for controlling active frequency droop of an AC system based on a line power flow controller comprises the following steps:

[0032] Step 1: Unlock the converter connected to the auxiliary control line of the line power flow controller.

[0033] This step is mainly to start the line power flow controllers. The line power flow controllers need to be started one by one. The reason why the line power flow controllers on the auxiliary control line are unlocked first is because the line power flow controllers on the auxiliary control line are used to control the DC voltage and need to be unlocked first.

[0034] Step 2: Set the power flow controllers between the auxiliary control lines to operate in constant DC voltage and constant reactive power control mode.

[0035] Set the DC voltage control mode and increase the DC voltage to the rated value. This will establish the DC side voltage and ensure stable operation of the line power flow controller.

[0036] Step 3: Unlock the converters connected to each main control line of the line power flow controller in turn.

[0037] After the DC voltage is established, the line-to-line power flow controllers on other master control lines can be unlocked, because the line-to-line power flow controllers on other master control lines are mainly used to control the active and reactive power of the line and cannot control the voltage on the DC side.

[0038] Step 4: Set the power flow controllers between the lines connected to each master control line to operate in the fixed AC line active power and fixed AC line reactive power control modes.

[0039] This step is to make each line operate at a given active and reactive power value.

[0040] Step 5: Extract the AC system frequency difference signal Δf at the receiving end of the main control line ac .

[0041] Because each main inter-line power flow controller can control the active power, it is considered to use this control capability to adjust the frequency stability of the receiving power grid, so it is necessary to first extract the frequency difference signal as the controller input.

[0042] Step 6: Extract the frequency difference signal Δf ac Multiplying by the droop coefficient K gives the active frequency droop signal.

[0043] The frequency signal is converted into a power signal through the droop coefficient for the main control power control.

[0044] Step 7: Add the active frequency droop signal to the active power control instruction of the line power flow controller connected to each master line to implement droop control based on the line power flow controller.

[0045] The converted power signal is added to the original power reference value to achieve a droop control effect.

[0046] by Figure 4 Take the IPFC AC transmission system as an example for verification:

[0047] Verification plan: Set the inter-line power flow controller 1 (IPFC1) as the auxiliary control line, and IPFC2 and IPFC3 as the main control lines. The relevant control methods are shown in Table 1.

[0048] Table 1 Main control line related control mode

[0049]

[0050] according to Figure 5 As shown in the block diagram, the droop control strategy is set in the IPFC2 and IPFC3 fixed active power links. Figure 5 Where f is the real-time frequency measured by the receiving AC system, 50Hz is the rated frequency of the AC system, and K droop is the droop coefficient, which is set to 9000. Pset is the active power setting value of IPFC2 and IPFC3, which are 200MW and 150MW respectively. ref It is the final power command output by IPFC.

[0051] In the case of system load fluctuations, the frequency fluctuation comparison diagram with and without the above droop strategy is as follows Figure 6 and Figure 7 It can be seen that droop control can not only improve the system frequency stability and reduce the frequency fluctuation range, but also enhance the IPFC's ability to support the AC system frequency and prevent system instability.

Claims

1. A method for controlling active frequency droop of an AC system based on a line power flow controller, characterized in that: The following steps are involved: Step 1: Unlock the converter connected to the auxiliary control line of the line power flow controller; Step 2: Set the power flow controller between the auxiliary control lines to operate in constant DC voltage and constant reactive power control mode; Step 3: Unlock the converters connected to each main control line of the line power flow controller in turn; Step 4: Set the power flow controllers between the lines connected to each master control line to operate in the fixed AC line active power and fixed AC line reactive power control modes in sequence; Step 5: Extract the AC system frequency difference signal Δ at the receiving end of the main control line f ac ; Step 6: Extract the frequency difference signal Δ f ac Multiply by the droop coefficient K Obtain active frequency droop signal; Step 7: Add the active frequency droop signal to the active power control instruction of the line power flow controller connected to each master line to implement droop control based on the line power flow controller.

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