Decentralized Transient Stability Control Method for a Hybrid System of Grid-Following and Grid-Forming Converters

Abstract

The present invention discloses a decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters. Firstly, when a fault occurs in the hybrid system of grid-following and grid-forming converters is detected, the output phase of the phase-locked loop (PLL) of the grid-following converter and the output phase of the power synchronization loop (PSL) of the grid-forming converter are obtained. Then, the difference between the output phase of the PLL and the phase at steady state is used as the virtual angle of the grid-following converter, and the difference between the output phase of the PSL and the specified phase value is used as the virtual angle of the grid-forming converter. Furthermore, the active current output control commands and reactive current output control commands of the grid-following converter and the grid-forming converter are determined. The decentralized control of the present invention can not only effectively improve the transient synchronization stability of the hybrid system of grid-following and grid-forming converters, but also avoid the grid-forming converter being locked in the current-limiting mode after the fault is cleared, which has important significance in ensuring the safe and stable operation of the power system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic converter control for power conversion between AC and DC, 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 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, resulting in 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 ability. 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 ability.

[0003] However, due to the significant differences in control strategies and external characteristics between grid-following and grid-forming converters, and the strong non-linear characteristics introduced by control switching such as saturation limiting 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 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:

[0006] A decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters, comprising the following steps:

[0007] Step 1: After the grid-connected converter detects a fault in the hybrid system of the grid-following and grid-forming converters, it controls the startup and obtains the output phase of the phase-locked loop (PLL) of the grid-following converter and the output phase of the power synchronization loop (PSL) of the grid-forming converter;

[0008] Step 2: Based on the output phase of the phase-locked loop (PLL) of the grid-following converter obtained in Step 1, take the difference between it and the phase in the steady state as the virtual rotation angle, determine the active current output control command of the grid-following converter and the reactive current output control command of the grid-following converter, and complete the current closed-loop control of the grid-following converter;

[0009] Step 3: Based on the output phase of the power synchronization loop (PSL) of the grid-forming converter obtained in Step 1, take the difference between it and the specified phase value as the virtual rotation angle of the grid-forming converter, determine the active current output control command of the grid-forming converter and the reactive current output control command of the grid-forming converter, and complete the current closed-loop control of the grid-forming converter;

[0010] 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.

[0011] In the above Step 2, the active current output control command of the grid-following converter and the reactive current output control command of the grid-following converter are respectively:

[0012] ,

[0013] where, I Ld and I Lq are respectively the active current output control command of the grid-following converter and the reactive current output control command of the grid-following converter; I L is the current amplitude when the grid-following converter operates normally; θ and θ s are respectively the output phase of the phase-locked loop (PLL) and the steady-state phase.

[0014] In the above Step 3, the active current output control command of the grid-forming converter and the reactive current output control command of the grid-forming converter are respectively:

[0015] ,

[0016] where, I Md and I Mq are respectively the active current output control command of the grid-forming converter and the reactive current output control command of the grid-forming converter; IM is the current amplitude when the grid-forming converter operates normally; is the output phase of the power synchronization loop PSL; ε is the specified phase value, taking [- π / 2, - π / 4].

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] Currently, the control ideas for hybrid parallel grid-connected systems of heterogeneous converters such as grid-following and grid-forming 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 angle 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 is 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 output phase of 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 output phase of the power synchronization loop PSL and the specified phase value as a virtual rotation angle to adjust the output active and reactive current output control commands. Compared with the centralized or collaborative control strategies, the advantages of the present invention are: 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 phase-locked loop 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 practical value in ensuring the safe and stable operation of the power system. Description of the Drawings

[0019] Figure 1 is the flowchart of the method of the present invention.

[0020] Figure 2 is the control structure block diagram of the present invention.

[0021] Figure 3 is the topology of a hybrid parallel grid-connected system based on grid-following and grid-forming converters.

[0022] 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.

[0023] Figure 5It 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. Specific Embodiments

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] 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:

[0026] 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. 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 ;

[0027] 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 of the grid-following converter 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;

[0028] 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 of the grid-forming converter 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;

[0029] 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.

[0030] Embodiment

[0031] The method of the present invention aims at the decentralized transient stability problem of the hybrid system of 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 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 grid-following and grid-forming converters includes a grid-following converter, a grid-forming converter, a transmission line, and an equivalent infinite system as shown in Figure 3 . The main circuit electromagnetic transient model runs in real time in the RTDS, and the converter control algorithm is implemented in the DSP controller. Simulation verification is carried out according to the following parameters:

[0032] 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.

[0033] The fault is set as the infinite bus voltage drops to 0.01 p.u. and lasts for 200 ms.

[0034] As shown in Figure 4 , the hardware-in-the-loop test results of the controller show that: 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, so it can be judged that it finally stabilizes in the current-limiting mode; while 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, so it can be judged that it finally stabilizes in the constant voltage mode, verifying that the present invention can avoid the grid-forming converter being finally locked in the current-limiting mode. As shown in Figure 5 , the hardware-in-the-loop test results of the controller show that: 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 decentralized transient stability control method for a hybrid system of grid-following and grid-forming converters, characterized in that: 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 the start and obtains the output phase of the phase-locked loop (PLL) of the grid-following converter and the output phase of the power synchronization loop (PSL) of the grid-forming converter; Step 2: Based on the output phase of the PLL of the grid-following converter obtained in Step 1, taking the difference between it and the phase at steady state as the virtual rotation angle, determining the active current output control instruction and the reactive current output control instruction of the grid-following converter, and completing the current closed-loop control of the grid-following converter; Step 3: Based on the output phase of the PSL of the grid-forming converter obtained in Step 1, taking the difference between it and the specified phase value as the virtual rotation angle of the grid-forming converter, determining the active current output control instruction and the reactive current output control instruction of the grid-forming converter, and completing 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.

2. The decentralized transient stability control method for the hybrid series-parallel system of the follow-the-network type and network-forming type converters according to claim 1, characterized in that: In the said Step 2, the active current output control instruction and the reactive current output control instruction of the grid-following converter are respectively: ， Wherein, I Ld and I Lq are respectively the active current output control command of the grid - following converter 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 phase at steady state.

3. The decentralized transient stability control method for the hybrid series-parallel system of the network-following type and network-forming type converters according to claim 1, wherein: In the said Step 3, the active current output control instruction and the reactive current output control instruction of the grid-forming converter are respectively: ， Wherein, I Md and I Mq are the active current output control command of the network-forming converter and the reactive current output control command of the network-forming converter respectively; I M is the current amplitude when the network-forming converter operates normally; is the output phase of the power synchronization loop PSL; ε is the specified phase value, taking [- π / 2, - π / 4].

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