Distributed transient stability control method for network following type and network constructing type converter series-parallel system

Abstract

The invention discloses a distributed transient stability control method for a network-following and network-constructing type converter hybrid system, and the method comprises the steps: obtaining a phase-locked loop (PLL) output phase of a network-following type converter and a power synchronization loop (PSL) output phase of a network-constructing type converter under the condition that the network-following and network-constructing type converter hybrid system is detected to have a fault; and then, taking the difference between the output phase of a phase-locked loop (PLL) and the phase in a steady state as a virtual rotation angle of the grid-following type converter, and taking the difference between the output phase of a power synchronization loop (PSL) and a specified phase value as a virtual rotation angle of the grid-constructing type converter, thereby determining an active current output control instruction and a reactive current output control instruction of the grid-following type converter and the grid-constructing type converter. Through distributed control, the transient synchronization stability of the hybrid system of the network-following type converter and the network-constructing type converter can be effectively improved, meanwhile, the situation that the network-constructing type converter is locked in a current limiting mode after a fault is cleared is avoided, and the method has important significance in the aspect of guaranteeing safe and stable operation of a power system.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic converter control for power conversion between alternating current and direct current, and particularly relates to a decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters. Background Art

[0002] At present, the penetration rate of new energy sources represented by wind power and photovoltaic power in the power grid continues to climb, and the power system shows significant power electronic characteristics. The traditional new energy grid connection method dominated by grid-following converters based on the phase-locked loop (PLL) leads to the continuous weakening of the system inertia support ability, triggering new stability problems such as broadband oscillation and overvoltage crossing. The grid-forming converter, with its active voltage and frequency support ability and self-synchronization characteristics, provides a new technical path for improving the new energy consumption capacity. In engineering practice, the system architecture of hybrid connection of grid-following and grid-forming converters has become an important solution for improving the new energy transmission capacity.

[0003] However, due to the significant differences in control strategies and external characteristics between grid-following and grid-forming converters, combined with the strong non-linear characteristics introduced by control switching such as saturation limit and fault ride-through of the converters, the hybrid converter transmission system faces the risks of transient synchronization instability and the grid-forming converter being locked in the current-limiting mode after fault clearing. In addition, in practical applications, the coordinated control of multiple devices during the transient process faces real-time communication problems. Therefore, developing a decentralized stabilization control strategy that does not rely on real-time interactive communication has important engineering value for ensuring the large-scale and safe grid connection of new energy. Summary of the Invention

[0004] In order to overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters. The present invention adjusts the active and reactive power outputs of the grid-following converter and the grid-forming converter in real time through the feedback of the phase measured by the phase-locked loop (PLL) of the grid-following converter, and adjusts the active and reactive power outputs of the grid-forming converter in real time through the feedback of the phase measured by the power synchronization loop (PSL) of the grid-forming converter, thereby improving the transient synchronization stability of the new energy transmission system without relying on real-time interactive communication and solving the problem that the first-order grid-forming converter is locked in the current-limiting mode after fault clearing.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows: A decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters, comprising the following steps: Step 1: After the grid-connected converter detects a fault in the hybrid system of grid-following and grid-forming converters, it controls to start, and obtains the phase output by the phase-locked loop (PLL) of the grid-following converter and the phase output by the power synchronization loop (PSL) of the grid-forming converter; Step 2: Take the difference between the phase output by the phase-locked loop (PLL) of the grid-following converter obtained in Step 1 and the phase at steady state as the virtual rotation angle, determine the active current output control command and the reactive current output control command of the grid-following converter, and complete the current closed-loop control of the grid-following converter; Step 3: Take the difference between the phase output by the power synchronization loop (PSL) of the grid-forming converter obtained in Step 1 and the specified phase value as the virtual rotation angle of the grid-forming converter, determine the active current output control command and the reactive current output control command of the grid-forming converter, and complete the current closed-loop control of the grid-forming converter; Step 4: When the control time is greater than the requirement of the new energy active recovery time limit, the control exits; otherwise, return to Step 2.

[0006] In the above-mentioned Step 2, the active current output control command and the reactive current output control command of the grid-following converter are respectively: , wherein, I Ld and I Lq are respectively the active current output control command and the reactive current output control command of the grid-following converter; I L is the current amplitude during the normal operation of the grid-following converter; θ and θ s are respectively the phase output by the phase-locked loop (PLL) and the steady-state phase.

[0007] In the above-mentioned Step 3, the active current output control command and the reactive current output control command of the grid-forming converter are respectively: , wherein, I Md and I Mq are respectively the active current output control command and the reactive current output control command of the grid-forming converter; I M is the current amplitude during the normal operation of the grid-forming converter; is the phase output by the power synchronization loop (PSL); ε is the specified phase value, taking [- π / 2, - π / 4].

[0008] Compared with the prior art, the present invention has the following advantages: The current control ideas for hybrid grid-connected systems of heterogeneous converters such as grid-following and grid-forming converters mainly focus on the collaborative control optimization of the current limiting angles of heterogeneous converters and the adaptive adjustment of power reference values. During a fault ride-through, although changing the collaborative control of the limiting angles can effectively minimize the transient energy accumulation of heterogeneous converters, it usually relies on real-time communication between heterogeneous converters, which is difficult to achieve during the extremely short fault transient period. The present invention provides a decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters. Through the state variables output by each converter itself for internal closed-loop feedback, the grid-following converter uses the difference between the phase output by the phase-locked loop (PLL) and the phase at steady state as a virtual rotation angle to adjust the active and reactive current output control commands; the grid-forming converter uses the difference between the phase output by the power synchronization loop (PSL) and a specified phase value as a virtual rotation angle to adjust the output active and reactive current output control commands. Compared with centralized or collaborative control strategies, the advantages of the present invention are as follows: 1) Dynamic decoupling between heterogeneous converters is achieved through local state variable feedback, facilitating decentralized control; 2) The transient output of the grid-following converter is adjusted to provide equivalent damping for the PLL, thereby improving the transient synchronization stability of the grid-following converter; 3) The equilibrium point of the first-order grid-forming converter in the current limiting mode is eliminated through state feedback, thus avoiding the first-order grid-forming converter being locked in the current limiting mode and unable to exit. In addition, the present invention has few measurement quantities, simple control implementation, and high reliability, and has engineering practice value in ensuring the safe and stable operation of the power system. Description of the Drawings

[0009] Figure 1 is the flowchart of the method of the present invention.

[0010] Figure 2 is the control structure block diagram of the present invention.

[0011] Figure 3 is based on the topology of a hybrid grid-connected system of grid-following and grid-forming converters.

[0012] Figure 4 is the waveform diagram of the active current of the grid-forming converter changing with the output phase of the grid-forming converter before and after adopting the control strategy of the present invention.

[0013] Figure 5 is the waveform diagram of the output phase of the grid-following converter changing with time before and after adopting the control strategy of the present invention. Detailed Implementation Modes

[0014] The present invention will be further described in detail below with reference to the drawings and embodiments.

[0015] As Figure 1 shown, the present invention is a decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters, including the following steps: Step 1: After the grid-connected converter detects a fault in the hybrid system of the grid-following and grid-forming converters, it controls to start. As Figure 2 shown, obtain the output phase of the phase-locked loop (PLL) of the grid-following converter θ and the output phase of the power synchronization loop (PSL) of the grid-forming converter ; Step 2: According to the output phase of the phase-locked loop (PLL) of the grid-following converter obtained in Step 1 θ , as Figure 2 shown, take the difference between it and the phase at steady state θ s as the virtual rotation angle η L , determine the active current output control instruction I Ld and the reactive current output control instruction I Lq of the grid-following converter, and complete the current closed-loop control of the grid-following converter; Step 3: According to the output phase of the power synchronization loop (PSL) of the grid-forming converter obtained in Step 1, as Figure 2 shown, take the difference between it and the specified phase value ε as the virtual rotation angle η M of the grid-forming converter, determine the active current output control instruction I Md and the reactive current output control instruction I Mq of the grid-forming converter, and complete the current closed-loop control of the grid-forming converter; Step 4: When the control time is greater than the requirement of the new energy active power recovery time limit, the control exits, otherwise return to Step 2.

[0016] Embodiment The method of the present invention aims at the decentralized transient stability problem of the hybrid system of the grid-following and grid-forming converters. To verify the correctness of the above analysis, a hardware-in-the-loop test platform, namely the hybrid grid-connected system of the grid-following and grid-forming converters, is constructed using an RTDS real-time digital simulator and a TMS320F28377D type DSP controller. Among them, the main circuit topology of the hybrid grid-connected system of the grid-following and grid-forming converters includes a grid-following converter, a grid-forming converter, a transmission line, and an equivalent infinite bus system as Figure 3 shown. The main circuit electromagnetic transient model runs in real time in the RTDS, and the converter control algorithm is implemented in the DSP controller. The simulation verification is carried out according to the following parameters: The actual output of the grid-following converter is 90 MW; the actual output of the grid-forming converter is 40 MW; the voltage level of the transmission line is 220 kV, and the rated frequency is 50 Hz. The control parameters are selected as - π / 2。

[0017] The fault is set such that the infinite bus voltage drops to 0.01 p.u. and lasts for 200 ms.

[0018] As Figure 4 shown by the test results of the controller hardware in the loop, when the control method designed by the present invention is not adopted, the δ = 5.76 rad after the grid-forming converter stabilizes, and the difference from the initial power angle is less than 2π. Therefore, it can be judged that it finally stabilizes in the current-limiting mode. After adopting the control designed by the present invention, the δ = 7.1 rad after the grid-forming converter stabilizes, and the difference from the initial power angle reaches 2π. Therefore, it can be judged that it finally stabilizes in the constant voltage mode, verifying that the present invention can prevent the grid-forming converter from finally locking in the current-limiting mode. As Figure 5 shown by the test results of the controller hardware in the loop, after adopting the control method designed by the present invention, the oscillation degree of the output phase of the grid-following converter is significantly reduced, proving the effectiveness of the control method designed by the present invention in improving the transient stability of the hybrid system.

Claims

1. A distributed transient stability control method for a grid-following and grid-building converter hybrid system, characterized in that: The following steps are involved: Step 1: After the grid-connected converter detects that a fault occurs in the hybrid system of the grid-following and grid-forming converters, the control starts to obtain the phase-locked loop PLL output phase of the grid-following converter and the power synchronization loop PSL output phase of the grid-forming converter; Step 2: According to the phase-locked loop PLL output phase of the grid-following converter obtained in step 1, the difference between the phase and the phase in the steady state is used as the virtual angle to determine the active current output control instruction of the grid-following converter and the reactive current output control instruction of the grid-following converter, and complete the current closed-loop control of the grid-following converter; Step 3: According to the power synchronization loop PSL output phase of the grid-type converter obtained in step 1, the difference between the output phase and the specified phase value is used as the virtual rotation angle of the grid-type converter, and the active current output control instruction and the reactive current output control instruction of the grid-type converter are determined to complete the current closed-loop control of the grid-type converter; Step 4: When the control time is greater than the required time limit for the active power recovery of new energy, the control exits, otherwise returns to step 2.

2. The distributed transient stability control method for a grid-following and grid-building converter hybrid system according to claim 1, characterized in that: In the step 2, the grid-following converter active current output control instruction and the grid-following converter reactive current output control instruction are respectively: ， In the formula, I Ld and I Lq They are respectively an active current output control instruction of a grid-following converter and a reactive current output control instruction of a grid-following converter; I L is the current amplitude of the grid-following converter during normal operation; θ and θ s They are the phase of the phase-locked loop PLL output and the phase in steady state respectively.

3. The distributed transient stability control method for a grid-following and grid-building converter hybrid system according to claim 1, characterized in that: In the step 3, the active current output control instruction of the grid-forming converter and the reactive current output control instruction of the grid-forming converter are respectively: ， In the formula, I Md and I Mq They are respectively an active current output control instruction of a grid-forming converter and a reactive current output control instruction of a grid-forming converter; I M is the current amplitude of the grid-type converter during normal operation; Output phase of power synchronization loop PSL; ε To specify the phase value, take [- π / 2,- π / 4].

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