Frequency support control method of ultra-high voltage direct current system for improving new energy consumption capacity

Abstract

The present application relates to a kind of new energy consumption capacity's UHVDC system frequency support control method of promotion, belong to UHVDC transmission field.The present application is aimed at the size of the unbalanced power after the disturbance of sending terminal system, the frequency support demand of different working conditions is analyzed;For the difference of response speed and support capacity of synchronous machine and UHVDC, design the multi-time scale frequency modulation control strategy of UHVDC system;For the maximum reserve capacity of synchronous machine primary frequency modulation, the dead zone value calculation method of multi-time scale frequency modulation control strategy is proposed.The frequency instability problem caused by the high proportion of new energy in the sending terminal grid of UHVDC system is solved, the rational use of frequency modulation resource is realized, and the frequency stability of the sending terminal grid of UHVDC system is improved.It has the advantages of scientific and reasonable, strong applicability, high reliability and good effect.

Description

Technical Field

[0001] This invention relates to the field of ultra-high voltage direct current (UHVDC) power transmission, and in particular to a frequency support control method for UHVDC systems that enhances the absorption capacity of new energy sources. Background Technology

[0002] With the proposal of the "dual carbon" target and the establishment of the strategy of "building a new power system with new energy as the main body", the large-scale development and utilization of new energy is the future trend. Line commutated converter-high voltage direct current (LCC-HVDC) transmission technology has the characteristics of low loss, low cost, and high reliability, which can significantly improve the long-distance, large-capacity transmission capacity of the power grid and is an effective means of connecting large-scale new energy bases to the grid for power transmission.

[0003] As the proportion of renewable energy in the sending-end system of ultra-high voltage direct current (UHVDC) transmission gradually increases, the hybrid AC / DC power grid structure is becoming increasingly prominent, leading to problems such as reduced inertia and decreased frequency regulation capability in UHVDC transmission systems. The available frequency regulation resources within the sending-end system mainly include synchronous generators, renewable energy sources, and the UHVDC system itself. However, due to the intermittent, uncertain, and random characteristics of renewable energy sources such as wind and solar power, their participation in system frequency regulation usually requires a certain reserve capacity or energy storage. Frequency regulation capability is significantly affected by operating conditions and suffers from high regulation costs and complex coordination. To address the frequency instability problem caused by the high proportion of renewable energy sources, the Frequency Limit Controller (FLC) has unique advantages in frequency stability control and has been widely used in the Wudongde DC project and the Siasun-Dongfang ±800kV UHVDC project. Properly configuring its parameters can effectively control the system frequency and improve the periodic oscillation problem of the system frequency. However, current research only considers the case where the UHVDC system only provides inertia support when the synchronous generator frequency regulation capacity is sufficient, and does not consider the case where the synchronous generator frequency regulation capacity is insufficient. For large-scale renewable energy transmission systems with low inertia and poor frequency regulation capabilities, further exploring the regulation potential of ultra-high voltage direct current systems and designing a multi-timescale coordinated control strategy that allows them to actively participate in the frequency regulation of the sending-end system will effectively improve the renewable energy absorption capacity and ensure maximum renewable energy output while maintaining stable frequency of the sending-end system. Summary of the Invention

[0004] The purpose of this invention is to provide a frequency support control method for ultra-high voltage direct current (UHVDC) systems that enhances the absorption capacity of renewable energy, addressing the frequency instability problem caused by the high proportion of renewable energy in the sending-end grid of existing UHVDC systems. This invention further explores the regulation potential of UHVDC systems, proposing a multi-timescale coordinated control strategy to actively support the frequency of the sending-end system, improving the frequency stability of the sending-end grid. This strategy is scientifically sound, highly applicable, reliable, and effective. It is of great significance for large-scale renewable energy transmission systems with low inertia and poor frequency regulation capabilities.

[0005] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0006] A frequency support control method for ultra-high voltage direct current (UHVDC) systems to enhance the absorption capacity of new energy sources includes the following steps:

[0007] Step 1: Analyze the frequency support requirements under different operating conditions based on the magnitude of the unbalanced power after the sending-end system is disturbed;

[0008] Step 2: To address the differences in response speed and support capabilities between synchronous machines and UHVDC systems, a multi-time-scale frequency modulation control strategy for the UHVDC system is designed.

[0009] Step 3: For the maximum reserve capacity of the synchronous machine's primary frequency regulation, a method for calculating the dead zone value of the multi-time-scale frequency regulation control strategy is proposed.

[0010] The frequency support requirements for different operating conditions mentioned in step 1 are as follows: when the backup capacity of the synchronous machine can completely absorb the unbalanced power of the sending-end AC system, the frequency regulation task of the sending-end system is undertaken solely by the synchronous machine; when the synchronous machine of the sending-end AC system cannot completely regulate the unbalanced power, the frequency regulation task of the sending-end system is jointly undertaken by the synchronous machine and the DC system.

[0011] The multi-time-scale frequency regulation control strategy for UHVDC systems described in step 2 combines the characteristics of fast response speed of UHVDC inertia support and strong synchronous machine regulation capability, and introduces the frequency change rate and frequency deviation into the DC active power control link.

[0012] The dead zone calculation method of the multi-time-scale frequency modulation control strategy described in step 3 determines the dead zone value of the multi-time-scale frequency modulation control strategy based on the maximum reserve capacity of the synchronous machine's primary frequency modulation. This enables the system to quickly and actively support the system frequency after power disturbances, and simultaneously possesses the ability to adaptively switch between inertia support and primary frequency modulation.

[0013] The beneficial effects of this invention are: it solves the frequency instability problem caused by the high proportion of new energy in the sending-end power grid of the UHVDC system, realizes the rational utilization of frequency regulation resources, and improves the frequency stability of the sending-end power grid of the UHVDC system. It has the advantages of being scientifically sound, highly applicable, highly reliable, and effective. Attached Figure Description

[0014] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate the invention and are used to explain it, but do not constitute an undue limitation of the invention.

[0015] Figure 1 This is a diagram illustrating the frequency coordination control strategy of the ultra-high voltage direct current transmission system of the present invention. Detailed Implementation

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0017] See Figure 1 As shown, the frequency support control method for UHVDC systems to enhance renewable energy absorption capacity, as presented in this invention, addresses the problems of increasing renewable energy penetration in the sending-end grid of UHVDC transmission systems, significantly reduced system inertia, and poor frequency regulation capability. It analyzes the frequency support requirements under different system operating conditions; proposes a multi-time-scale frequency regulation control strategy coordinating UHVDC and synchronous machines; and proposes a dead-zone value calculation method for the multi-time-scale frequency regulation control strategy. This solves the frequency instability problem caused by the high proportion of renewable energy in the sending-end grid of UHVDC systems, achieving rational utilization of frequency regulation resources and improving the frequency stability of the sending-end grid of UHVDC systems. It has the advantages of being scientifically sound, highly applicable, highly reliable, and effective.

[0018] The following description, in conjunction with the accompanying drawings, further illustrates the detailed content of the present invention and its specific embodiments.

[0019] Based on the maximum reserve capacity and unbalanced power of the primary frequency modulation of the sending-end AC system, its frequency support requirements are divided into the following two operating conditions:

[0020] Condition 1: If the unbalanced power of the sending-end AC system satisfies the constraint of equation (1) after the power disturbance, that is, the standby capacity of the synchronizing machine can completely absorb the unbalanced power, then the frequency regulation task of the sending-end system can be undertaken by the synchronizing machine alone.

[0021] ΔP unb ≤ΔP max (1)

[0022] In the formula, ΔP max It is the maximum standby capacity of the synchronous machine during a single frequency modulation, ΔP unb It refers to the unbalanced power of an AC system.

[0023] Under this condition, the unbalanced power when the frequency reaches steady state is entirely borne by the synchronous machine, and the UHVDC system only exhibits an inertial support function.

[0024] Condition 2: If the unbalanced power of the sending-end AC system satisfies the constraint of equation (2) after the power disturbance, that is, the frequency regulation of the sending-end AC system cannot completely regulate the unbalanced power by the primary frequency regulation of the synchronous machine. In this case, the frequency regulation task of the sending-end system is jointly undertaken by the synchronous machine and the DC system.

[0025] ΔP unb >ΔP max (2)

[0026] In this scenario, the unbalanced power when the frequency reaches steady state is shared by the synchronous machine and the DC motor. The UHVDC system plays the role of inertia support and primary frequency regulation in this process.

[0027] To address the frequency support requirements of the sending-end AC system under two operating conditions, and leveraging the advantages of the UHVDC system's fast regulation speed and strong synchronous machine frequency modulation capability, a multi-time-scale frequency modulation control strategy coordinating both is designed (see [reference]). Figure 1 As shown, it mainly consists of two parts: control loop 1 and control loop 2. The principle expression of the multi-time-scale frequency modulation control strategy is shown below.

[0028]

[0029] In the formula, f is the frequency of the AC system, f H It is the dead zone value, f ref This is the frequency rating, and Δf is the frequency deviation; K p It is the proportional gain, K i It is the integral gain, K d X is the differential adjustment coefficient. i It is a DC system state variable; ΔP i P is the change in DC power caused by control loop 1 after the frequency deviation exceeds the dead zone value. ref The rated power is given by ΔP0, which is the change in DC power caused by control loop 2. It is the output power of a DC system.

[0030] The specific working principle is described using the example of frequency rise after a disturbance in the sending-end system. Under this condition, the limiting circuit ensures that the DC system state variables x2 and ΔP2 remain zero. After a power disturbance, control circuit 2 rapidly increases the active power of the UHVDC system to hinder the frequency rise. Once the dead zone is exceeded, control circuit 1 also begins to regulate the unbalanced power, thereby suppressing the rate and magnitude of frequency rise. When the frequency drops back to the dead zone, the UHVDC regulation gradually decreases until all regulation is released, at which point the system exits operation.

[0031] Primary frequency regulation is automatically completed under the action of the prime mover speed control system, and the unbalanced power of the primary frequency regulation response is generally limited to within 6% of the rated load. Its regulation principle is shown in the following equation:

[0032] ΔP G =K G ·Δf (4)

[0033] In the formula, K G Δf is the adjustment coefficient for primary frequency modulation, ΔP is the frequency deviation of the current system, and ΔP is the frequency control factor. G The unbalanced power regulated by primary frequency regulation. The dead zone value of the multi-time-scale frequency regulation control strategy of the UHVDC system is determined by the maximum reserve capacity of the synchronous machine for primary frequency regulation, as shown in equation (5).

[0034] Δf0=ΔP max ·B p (5)

[0035] In the formula, B p The droop coefficient for primary frequency modulation, ΔP max Let f0 be the maximum reserve capacity of the primary frequency modulation at the sending end, and Δf0 be the frequency deviation corresponding to the maximum reserve capacity, which is the dead zone value f in this paper. H The frequency dynamic response process of the synchronous machine and the UHVDC system in multi-timescale coordination is as follows: The coordination principle between the synchronous machine and the UHVDC system is shown in equation (6).

[0036]

[0037] In the formula, H is the inertial time constant. It is the regulation output of the multi-timescale frequency modulation control strategy of the UHVDC system. The unbalanced power regulated by the load is negligible. The dynamic frequency response process of the system can be divided into two stages. The first stage is from power disturbance to frequency reaching its highest point. After the power disturbance, the frequency rises rapidly and the frequency change rate is the largest at the moment of disturbance. Under the action of control link 2, the UHVDC system will provide a large amount of power support in the early stage of disturbance. The output power at this time is as shown in equation (7):

[0038]

[0039] In the formula, P ref For the rated power, ΔP0 is the change in DC power caused by control link 2. When the frequency reaches the dead zone of the primary frequency regulation, the primary frequency regulation starts to operate and the synchronous machine reduces its output. After the frequency exceeds the dead zone value of control link 1, under the action of control link 1, the UHVDC system further adjusts its output power as shown in equation (8) until the frequency reaches the maximum value.

[0040]

[0041] In the second stage, the frequency deviation decreases to the dead zone value f of control loop 1. H The frequency oscillation recovery phase occurs within the specified range. After the frequency reaches its maximum value, since it is outside the dead zone of control loop 1, the DC regulation continues to increase under the multi-timescale frequency regulation control strategy of the UHVDC system. The synchronous machine continues to participate in the system's unbalanced power regulation, and the frequency begins to decrease. Once the frequency drops to the dead zone, the DC regulation continues to decrease, which may cause the frequency to rise again and exceed the dead zone. The subsequent dynamic process repeats the above process. When the frequency no longer exceeds the dead zone value under the action of primary frequency regulation, control loop 1 exits operation and actively exits control loop 2. At this time, the UHVDC output power regulation becomes 0, restoring the original operating state until the system frequency reaches a new stable state.

[0042] During this process, when the rate of change of frequency is positive, control loop 2 provides a positive active power increment.

[0043]

[0044] In the formula, K is the rate of change of frequency. d The differential adjustment coefficient is used to provide a negative active power increment when the frequency change rate is negative.

[0045]

[0046] After the frequency reaches its highest point, the frequency change rate becomes negative. Control loop 2 provides a negative active power increment, which slows down the rise of UHVDC transmission power and also accelerates the recovery of DC tie line transmission power, preventing it from being overloaded for a long time and damaging the switching devices.

[0047] As explained above, this invention enables the UHVDC system and synchronous generator units to coordinate in timing throughout the entire frequency disturbance process, effectively combining the control advantages of both. This alleviates the pressure of traditional DC regulation while achieving rapid and proactive frequency support, thus solving the frequency instability problem caused by the high proportion of new energy in the sending-end power grid of the UHVDC system.

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made to the present invention should be included within the scope of protection of the present invention.

Claims

1. A frequency support control method for an ultra-high voltage direct current system to enhance the absorption capacity of new energy sources, characterized in that: Includes the following steps: Step 1: Analyze the frequency support requirements under different operating conditions based on the magnitude of the unbalanced power after the sending-end system is disturbed. The frequency support requirements under different operating conditions are as follows: when the backup capacity of the synchronous machine can completely absorb the unbalanced power of the sending-end AC system, the frequency regulation task of the sending-end system is undertaken solely by the synchronous machine; when the synchronous machine of the sending-end AC system cannot completely regulate the unbalanced power, the frequency regulation task of the sending-end system is jointly undertaken by the synchronous machine and the DC system. Step 2: To address the differences in response speed and support capability between synchronous machines and UHVDC, a multi-time-scale frequency modulation control strategy for the UHVDC system is designed; taking into account the characteristics of the UHVDC system's fast inertial support response speed and the strong control capability of synchronous machines, the frequency change rate and frequency deviation are introduced into the DC active power control loop. Step 3: For the maximum reserve capacity of the primary frequency regulation of the synchronous machine, a method for calculating the dead zone value of the multi-time-scale frequency regulation control strategy is proposed. The dead zone value of the multi-time-scale frequency regulation control strategy is determined by the maximum reserve capacity of the primary frequency regulation of the synchronous machine, so that the system can quickly and actively support the system frequency after power disturbance, and at the same time have the ability to adaptively switch between inertia support and primary frequency regulation. Based on the maximum reserve capacity and unbalanced power of the primary frequency modulation of the sending-end AC system, its frequency support requirements are divided into the following two operating conditions: Condition 1: If the unbalanced power of the sending-end AC system satisfies the constraint of equation (1) after the power disturbance, that is, the standby capacity of the synchronizing machine can completely absorb the unbalanced power, then the frequency regulation task of the sending-end system is only undertaken by the synchronizing machine. （1）； In the formula, It is the maximum standby capacity of a single frequency modulation of a synchronous machine. It is the unbalanced power of the AC system; Condition 2: If the unbalanced power of the sending-end AC system satisfies the constraint of equation (2) after the power disturbance, that is, the frequency regulation of the sending-end AC system cannot completely regulate the unbalanced power by the primary frequency regulation of the synchronous machine. In this case, the frequency regulation task of the sending-end system is jointly undertaken by the synchronous machine and the DC. （2）； To address the frequency support requirements of the sending-end AC system under two operating conditions, and leveraging the advantages of the UHVDC system's fast regulation speed and strong synchronous machine frequency modulation capability, a multi-time-scale frequency modulation control strategy coordinating the two is designed. This strategy comprises two parts: control loop 1 and control loop 2. The expression for the multi-time-scale frequency modulation control strategy is shown below: （3）； In the formula, It is the frequency of the AC system. It is the dead zone value. It is the frequency rating. This is for frequency deviation; It is proportional gain. It is integral gain. The differential adjustment coefficient is... It is a DC system state quantity; It is the change in DC power caused by control loop 1 after the frequency deviation exceeds the dead zone value. Rated power, It is the change in DC power caused by control loop 2. It is the output power of a DC system; It is the maximum state variable of the DC system; Primary frequency regulation is automatically completed under the action of the prime mover speed control system. The unbalanced power of the primary frequency regulation response is generally limited to within 6% of the rated load. Its regulation principle is shown in the following formula: （4）； In the formula, This is the adjustment coefficient for primary frequency modulation. The current system frequency deviation, The unbalanced power regulated by primary frequency regulation; the dead zone value of the multi-time-scale frequency regulation control strategy of the UHVDC system is determined by the maximum reserve capacity of the primary frequency regulation of the synchronous machine, as shown in equation (5). （5）； In the formula, This is the droop coefficient for primary frequency modulation. This is the maximum reserve capacity for primary frequency modulation at the sending end. The frequency deviation corresponding to the maximum reserve capacity of frequency modulation is the dead zone value. The frequency dynamic response process of the synchronous machine and the UHVDC system in multi-timescale coordination is as follows: The coordination principle between the synchronous machine and the UHVDC system is shown in equation (6): （6）； In the formula, The inertial time constant, It is the adjustment quantity output by the multi-time-scale frequency modulation control strategy of the UHVDC system; the unbalanced power regulated by the load is negligible; the dynamic frequency response process of the system can be divided into two stages. In the first stage, the power disturbance reaches the frequency at its highest point. After the power disturbance, the frequency rises rapidly and the frequency change rate is the largest at the moment of disturbance. Under the action of control link 2, the UHVDC system will provide a large amount of power support in the early stage of disturbance. The output power at this time is as shown in equation (7): （7）； In the formula, Rated power, The change in DC power caused by control link 2. When the frequency reaches the dead zone of the first frequency modulation, the first frequency modulation starts to operate and the synchronous machine reduces the output. After the frequency exceeds the dead zone value of control link 1, under the action of control link 1, the UHVDC system further adjusts its output power, as shown in equation (8), until the frequency reaches the maximum value. （8）； In the second stage, the frequency deviation decreases to the dead zone value of control loop 1. The frequency oscillation recovery phase is within the range; after the frequency reaches its maximum value, since the frequency is outside the dead zone of control loop 1, the DC regulation continues to increase under the action of the multi-time-scale frequency regulation control strategy of the UHVDC system, the synchronous machine continues to participate in the unbalanced power regulation of the system, and the frequency begins to drop; after the frequency drops to the dead zone, the DC regulation continues to decrease, which may cause the frequency to rise again and exceed the dead zone; the subsequent dynamic process repeats the above process, and after the frequency no longer exceeds the dead zone value under the action of primary frequency regulation, control loop 1 exits operation and actively exits control loop 2. At this time, the UHVDC output power regulation becomes 0, and the original operating state is restored until the system frequency reaches a new stability; During this process, when the rate of change of frequency is positive, control loop 2 provides a positive active power increment; （9）； In the formula, The rate of change of frequency, The differential adjustment coefficient is used so that when the rate of change of frequency is negative, control loop 2 provides a negative active power increment. （10）； After the frequency reaches its highest point, the frequency change rate becomes negative. Control loop 2 provides a negative active power increment, which slows down the rise of UHVDC transmission power and also accelerates the recovery of DC tie line transmission power, preventing it from being overloaded for a long time and damaging the switching devices.

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