Transient stability emergency control for AC/DC grids considering successive commutation failures

This paper proposes a safe reinforcement learning (RL)-based transient stability emergency control (TSEC) method for islanded microgrids. RL requires extensive interaction with the environment to learn control strategies, hence, a data-driven approach is used as a substitute for time-consuming time-domain simulation calculations. Deep sigma point processes (DSPP), which is a Gaussian process model, is utilized to predict the normal distribution of transient stability of microgrids and to construct a transient stability chance constraint. Reward-constrained policy optimization (RCPO) can simultaneously achieve objective prediction, policy learning, and constraint cost coefficient update across multiple timescales. RCPO interacts with the DSPP-based microgrid environment through a multi-process parallel manner, greatly increasing the training speed. Case studies on a real islanded microgrid demonstrate that the proposed method can efficiently and quickly obtain the optimal emergency control strategy while adhering to all hard constraints.

An on-line transient stability emergency control approach is proposed, which couples an open-loop and a closed-loop emergency control technique. The open-loop technique uses on-line transient stability assessment in order to adapt the settings of automatic system protection schemes to the current operating conditions. On the other hand, the closed-loop technique uses measurements in order to design and trigger countermeasures, after the contingency has actually happened, then to continue monitoring in a closed-loop fashion. The approach aims at combining advantages of event-based and measurement-based system protection schemes, namely, speed of action and robustness with respect to uncertainties in system modeling. It can also comply with economic criteria.

Based on the hybrid HVDC transmission system, which is composed by line‐commutated converter (LCC) in parallel with voltage source converter (VSC), a coordinated reactive power control strategy of hybrid‐HVDC in both steady and transient states is proposed. The operation characteristics in situations like low load operation, filter switching, and de‐locking process are analysed. Influence of the method to AC bus voltage and system power factor is taken into account. Three kinds of reactive control methods of VSC and LCC are studied, and the logical relationship between the filter and VSC reactive power controller is also given. Considering both the system voltage drop and the risk of commutation failure caused by AC faults, the VSC in voltage control mode is utilised to maintain AC bus voltage stability by providing emergency power support. Simulation results show the feasibility and effectiveness of presented control strategy, then the proposed strategy is applied in practical DC project, whose results achieve the desired effect by engineering field test.

Grid Integration of offshore wind farms using a Hybrid HVDC composed by an MMC with an LCC-based transmission system

High-voltage dc (HVDC) transmission based on the line-commutated converter (LCC) is the most established and widespread technology around the world. However, HVDC based on the voltage source converter (VSC) has emerged as the best option to integrate renewable energy sources, e.g., offshore wind farms. This work investigates the feasibility of using a conventional LCC-HVDC transmission in combination with a VSC to integrate offshore wind farms. Such integration results in a hybrid HVDC connection, i.e., the connection of a VSC with an LCC through a dc cable. The operational features of a model of three-terminal hybrid HVDC, two LCC stations and one VSC station, is investigated using PSCAD/EMTDC. The simulations include an aggregate model to emulate the wind farm. The corresponding control strategies are proposed for each terminal and verified under various conditions including wind speed variations and ac faults.

Modifying the inverter of line-commutated converter- based high-voltage direct current (LCC-HVDC) to hybrid commutated converter (HCC) is currently a highly effective solution for mitigating the risk of commutation failure (CF). This paper investigates the transient performance of HCC from five perspectives while retaining the LCC-HVDC control system. Since constant extinction angle (CEA) controller leads to significant reduction in power transmission, constant voltage (CV) controller is considered a more appropriate choice as the main controller during AC faults. In addition, the extreme case of insufficient forced commutation capacity leading to CF is considered. It is revealed for the first time that the voltage clamping effect of the arrester will cause a new CF phenomenon. There will be no short circuit on the DC side and the fault current is greatly reduced. This leads to insufficient regulation capability of the CV controller, resulting in continuous CFs. Given the limited fault ride-through capability of the selected CV controller, this paper proposes an adaptive order regulation control (AORC) based on expected reactive power. It can adjust the DC current and voltage in a targeted manner according to AC faults, and determine the maximum allowable upper limit of the firing angle. This improves the reliability of HCC in resisting CF. Under extreme cases, it also effectively prevents the continuous CFs. The simulation results in PSCAD/EMTDC verify the theoretical analyses and the proposed control method.

Commutation failure is a serious malfunction in Line-Commutated High Voltage Direct Current (HVDC) converters which is mainly caused by the inverter AC faults, and results in a temporary interruption of transmitted power and damage to the converter equipment. In this article, a Controllable Commutation Failure Inhibitor (CCFI) is developed which obviates the main drawbacks of the existing power electronic based and fault current limiting based strategies. Under normal circumstances, the developed CCFI improves the steady-state stability and the power transfer capability of the inverter AC lines, while it does not cause excessive voltage stress on the converter valves. In addition, it would reduce the risk of commutation failure occurrence, since it does not lead to any voltage drop in the commutation circuit. When a fault occurs at one of the inverter AC systems, its corresponding CCFI limits the fault current depending on the reduced extinction angle. This would not only inhibit the successive commutation failures on the HVDC converter, but also extend the lifetime of components in the inverter AC systems. The practical feasibility of the developed CCFI is assessed through laboratory testing, using a real-time Opal-RT hardware prototyping platform. The obtained results indicate that the developed CCFI can reliably inhibit the commutation failures during various types of faults.

With the rapid development of ultra-high-voltage (UHV) AC/DC, especially the step-by-step upgrading of the UHV DC transmission scale, security presents new challenges. Commutation failure (CF) is a common fault in line commutated converter (LCC) high-voltage direct current (HVDC) power systems. Once failure happens, it may cause power oscillations in a system. In this paper, taking a two-area AC/DC parallel interconnected power system as the example, based on the impulse response model of second-order linear system, the mechanism of power oscillation on the AC tie-line caused by CF are clarified. It is proved that the peak value of the AC tie-line power oscillation is mainly determined by the DC power and the equivalent CF duration, the frequency and damping ratio of dominant area oscillation mode. Meanwhile, the peak time is mainly determined by the oscillation frequency. Finally, the correctness and effectiveness of the algorithms are verified by a simulation analysis of an extended IEEE-39-bus AC/DC parallel interconnected power system. These research results can provide a basis for the arrangement of the operating modes and the formulation of control measures for interconnected power grids.

The behaviors of transient phenomena in HVDC systems, including DC line faults, commutation failures caused by AC faults and missing firing are difficult to be identified automatically by the control systems, while the effective protection control for commutation failures depends on the rapid and correct identification of such faults. This paper proposes a method to identify different causes leading to commutation failures based on the wavelet transform. By using the technique of wavelet multi-resolution analysis (MRA), the transient signals generated by the faults are decomposed into different resolution levels. The features of each fault are extracted. Two auxiliary parameters are defined as the criteria for the identification, based on which four thresholds are set to distinguish different faults. Simulation results indicate that the proposed approach could make a definite identification of commutation failures from DC line faults and normal operations. Furthermore, the discriminations between AC short circuit faults and missing firing faults, which cause commutation failures, also could be obtained.

An improved reactive power control strategy for LCC-HVDC to mitigate sending end transient voltage disturbance caused by commutation failures

Conventional high-voltage direct current (HVDC) transmission systems are widely employed in large-capacity, long-distance power delivery scenarios. However, due to the reliance on thyristor-based line-commutated converter (LCC) lacking self-turn-off capability, commutation failures are likely to occur when the alternating current system is weak or subjected to disturbances. The static synchronous compensator (STATCOM), a shunt-connected reactive power compensation device, can inject or absorb reactive power to support the LCC-HVDC inverter side, thereby mitigating commutation failures. Nevertheless, traditional grid-following STATCOMs exhibit stability limitations under weak grid conditions, as their performance is inherently tied to grid strength. With the rapid integration of renewable energy sources, modern power systems increasingly exhibit "dual-high" characteristics, heightening instability risks for conventional STATCOM. In this study, grid-forming control technology is applied to STATCOM devices. Grid-forming control enhances stability in weak grids by emulating the rotor dynamics of synchronous generators to autonomously regulate voltage and frequency. A simulation platform integrating grid-forming STATCOM and LCC-HVDC systems is established in PSCAD/EMTDC. Case studies demonstrate the superior capability of grid-forming STATCOMs in suppressing commutation failures under weak grid conditions compared to conventional grid-following counterparts.

Small-signal stability of hybrid multi-terminal HVDC system

The commutation failure (CF) is the most common fault in line-commutated high voltage direct current (LCC-HVDC) systems that may lead to the transient overvoltage in the sending-side system. In the worst condition, the CF may lead to large-scale wind turbine tripping. To resolve this problem, the mathematical relationship between the reactive power consumed by the rectifier and DC voltage, DC current is derived. Then, a transient overvoltage calculation method is proposed in this paper. Furthermore, the mechanism of transient overvoltage caused by the CF is analyzed; it is revealed that the reason for three times transient overvoltage is the rapid decrease of the reactive power consumed by the rectifier during the CF and the recovery period from the CF. This paper proposes a constant reactive power control (CRPC) to inhibit transient overvoltage of the sending-side AC system. The proposed CRPC can increase the reactive power consumed by the rectifier, reduce the exchange reactive power between AC and DC systems, and suppress the transient overvoltage. A simulation model in PSCAD serves to verify the proposed CRPC on the transient overvoltage suppression in the situation of different fault types, fault duration, fault severity and short circuit ratio (SCR).

The increasingly prominent pattern of ac and hvdc hybrid connection and the continuous growth of new energy grid connected capacity have led to profound changes in the disturbed behavior and stability characteristics of the power grid. It is increasingly urgent to implement dynamic monitoring and stability control of large power grid based on wide area measurement system (WAMS). In this paper, firstly, the trajectory characteristics of the ac branch under the impact of large disturbances and the relationship between them and transient stability are studied. Secondly, the continuous tracking method of the branch voltage center based on the node voltage amplitude and phase is proposed, and the relationship between the branch voltage center and the system oscillation center is analyzed. Thirdly, for the problem that a single characteristic is necessary but not sufficient to identification the key branch, a method based on wide area response information and combined multiple characteristic parameters is proposed, furthermore, a transient stability emergency control strategy is put forward. Finally, the effectiveness of the key branch discrimination and control strategy is verified by simulation for ac and HVDC hybrid power grid.

A coordination control strategy to suppress commutation failure based on LCC-VSC