VDCOL Coordinated Control Strategy for Successive Commutation Failure Mitigation in Multi-infeed HVDC Systems

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

With the increasing integration of HVDC projects, multi-infeed HVDC systems are becoming more prevalent. The strong coupling between DC links can lead to successive commutation failures across multiple lines, known as cascading commutation failures. These failures cause significant transient power imbalances in both sending and receiving systems and may trigger voltage instability or even system-wide cascading faults due to insufficient reactive power support. To address this issue, this paper proposes a discrimination method for cascading commutation failures in AC/DC hybrid grids with multi-infeed HVDC systems. Based on a quasi-steady-state model of the DC system, the method analytically computes the voltage sag at converter buses, helping to reveal the mechanism of post-fault cascading failures in hybrid AC/DC systems. It enables rapid prediction of whether successive commutation failures will occur across multiple DC lines—without relying on specific grid operating conditions—and provides a theoretical foundation for subsequent control strategies. Finally, the method in this paper is verified by simulation in an actual system to prove its effectiveness.

To suppress the continuous commutation failure (CCF) and improve fault resilience of high voltage direct current (HVDC) due to alternative current (AC) fault occurred in inverter side, an improved voltage dependent current order limiter (VDCOL) control strategy considering real-time calculation of VDCOL starting voltage and virtual resistance comprehensively is proposed. By theoretical analysis, the traditional VDCOL starting voltage calculated by DC voltage fluctuates violently with long duration, which is a critical reason for CCF of HVDC after the first commutation failure (FCF). To reduce rapid shifts of conventional VDCOL starting voltage, an equation of VDCOL starting voltage characterized by AC voltage and DC current is established. In addition, considering transient characteristics of DC current under AC fault, virtual resistance is also introduced in mathematical equation of starting voltage. By integrating AC voltage, DC current and virtual resistance, VDCOL starting voltage which captures transient performance of AC / DC hybrid grid during fault occurrence and recovery stage can be calculated in real time. Based on CIGRE HVDC benchmark model, the proposed VDCOL control strategy is compared with the other two strategies in PSCAD/EMTDC. Comparison results verify that the improved VDCOL control method has the ability to mitigate CCF with a faster recovery time under various fault scenarios.

In the hybrid dual-infeed High Voltage Direct Current (HVDC) system, simulation methods are mainly used to study the inhibitory effect of Modular Multilevel Converter based HVDC (MMC-HVDC) on the commutation failure of Line Commutated Converter based HVDC (LCC-HVDC), but there is no mechanism analysis, which leads to a lack of universality. To solve the problem, this paper derives the expressions of LCC inverter bus voltage and short-circuit current after fault, quantitatively analyzes the support effect of MMC-HVDC on LCC inverter bus voltage, which provides theoretical support for MMC-HVDC to suppress the commutation failure of LCC-HVDC by providing reactive current. On this basis, a commutation failure suppression strategy for hybrid dual-infeed HVDC system is proposed. The reactive power compensation is obtained according to the required reduction of firing angle order and added to the MMC’s reactive-power outer control. This method can improve the reactive power support from MMC to ac system and suppress commutation failure. Finally, the mechanism analysis is proved correct and the control strategy is proved effective in PSCAD.

A commutation failure risk analysis method considering the interaction of inverter stations

Commutation failure is one of the most frequent failures in line-commutated converter based high-voltage direct-current (LCC-HVdc) systems which may lead to the outage of HVdc links, and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, a commutation failure prevention module (CFPM) is developed, which inserts the optimal value of resistance proportional to the reduced extinction angle in series with the fault current path using a simple switching method. When a fault occurs at the inverter ac side, a self turn- off switch starts switching with a pre-specified frequency and controlled modulation index, which limits the fault current to the desired value and inhibits commutation failure. The salient feature of the proposed CFPM is that it is fully controllable. In addition, despite the available controller modification methods which are not effective for low-impedance faults, the proposed CFPM can prevent commutation failure regardless of the fault severity. The practical performance and feasibility of the developed CFPM is validated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively eliminate the commutation failure or repetitive commutation failures under different fault types.

With increasing application of both Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) systems and Voltage Source Converter based HVDC (VSC-HVDC) links, a new type of system structure named Hybrid Multi-Infeed HVDC (HMIDC) system is formed in the modern power systems. This paper presents the operation and control method of the wind farm connected HMIDC system. The wind power fluctuation takes large influence to the system voltages. In order to reduce the voltage fluctuation of LCC-HVDC infeed bus caused by the wind power variation, a voltage sensitivity-based reactive power control method is proposed in the paper. According to the calculated sensitivity factors, a reactive power increment is added in the control loop of VSC-HVDC so as to regulate the voltage of the target bus. Dynamic simulations in PSCAD/EMTDC and MATLAB are presented to assess the performance of the proposed control method.

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.

The commutation failure (CF) mitigation effectiveness is normally restricted by the delay of extinction angle (EA) measurement or the errors of existing prediction methods for EA or firing angle (FA). For this purpose, this paper proposes a CF mitigation method based on the imaginary commutation process. For each sample point, an imaginary commutation process is constructed to simulate the actual commutation process. Then, the imaginary EA is calculated by comparing the imaginary supply voltage-time area and the imaginary demand voltage-time area, which can update the imaginary EA earlier than the measured EA. In addition, the proposed method considers the impacts of commutation voltage variation, DC current vari-ation, and phase angle shift of commutation voltage on the commutation process, which can ensure a more accurate EA calculation. Moreover, the DC current prediction is proposed to improve the CF mitigation performance under the single-phase AC faults. Finally, the simulation results based on CIGRE model prove that the proposed method has a good performance in CF mitigation.

The fault of the receiving-end AC power grid of the high voltage direct current (HVDC) transmission system may cause the HVDC subsequent commutation failures. In order to dynamically adjust the DC current command during the recovery process of the first HVDC commutation failure to achieve the purpose of suppressing the HVDC subsequent commutation failures, a new method is proposed based on improved genetic algorithm (IGA) of the voltage dependent current order limiter (VDCOL) link optimization method. This paper analyzes the influencing factors of HVDC subsequent commutation failures and the reactive power interaction between the inverter converter station and the external power grid. Taking VDCOL as the research object, the work principle and limitations of conventional VDCOL are introduced. In order to overcome the single linear defect of the conventional VDCOL link, an IGA was used to optimize the parameters of the multi-segment inflection point of the VDCOL link, which realized the dynamic reactive power demand adjustment of the inverter side converter station when the receiving-end AC power grid was faulty, and then suppress the HVDC subsequent commutation failures. Finally, the simulation results demonstrate the effectiveness of the proposed method.

Commutation failures (CFs) including repetitive CFs (RCFs) are frequently occurring faults in line commutated converter based high voltage direct current (LCC-HVDC) systems. Consequently, sending end system will suffer transient voltage disturbances (TVD) with a characteristic of “first reduce then rise and may repeat”. Due to the complex interaction process of TVD, the electromagnetic process behind this phenomenon cannot be easily explained, which imposes great challenges on the development of mitigation strategies. Given this, this paper provides a deep insight into the mechanism and the impact factors of the TVD caused by CFs and RCFs. The numerical relationship between the transmitting power and the TVD is derived. Research results revealed that the transient process after fault can be segmented into five stages. In each stage, the control interactions among HVDC controllers and power interactions at the PCC are analyzed. Besides, we discussed the TVD under RCFs in detail. Several indices are further proposed to facilitate the characterization of the TVD, and main control loops of HVDC that have impacts on the characteristics of TVD are thereby studied. Finally, the mathematical relationship between wind farm power and the TVD is derived to quantify the impact of wind farms on the TVD.

Multiple commutation failure suppression method of LCC-HVDC transmission system based on fault timing sequence characteristics

The receiving end system AC fault of the line-commutated-converter based high voltage direct current (LCC-HVDC) system will lead to commutation failure (CF) in inverter. During the fault and its recovery, AC transient low voltage and transient overvoltage (TOV) will occur in the sending end system. The TOV has the risk of triggering the disorderly off-grid of the nearby renewable power generations, and in a serious situation, it will threaten the power system can not maintain secure and steady operation. Therefore, this paper analyzes the mechanism involved of the AC transient voltage during the AC fault and the recovery period first. It reveals that the key factor causing the TOV of the sending end system is the setting of the DC current reference value. Then, a DC current reference value limit method based on the AC TOV sampling value is proposed, which is used to accelerate DC current recovery and suppress the TOV of sending end system. Finally, based on the CIGRE Benchmark test model, the transient voltage analysis results and the validity of the designed control method are demonstrated by electromagnetic transient simulation.

Sensitivity analysis of commutation failure in multi-infeed HVDC systems: Exploring the impact of ac system representations and fault types

Commutation failure predictive control (CFPREV) is widely used to suppress commutation failure (CF) in practical multi-feed DC systems (MIDC). The current research is mainly limited to the improvement of CF suppression strategy for single-loop DC system without considering the influence of this DC controller on other DC systems. The CFPREV not only reduces the risk of CF, but also increases the reactive power consumption of the local DC system, which will affect the normal commutation process of other DC systems. Therefore, a coordinated control method of MIDC for CFPREV is proposed in this paper. First, the reactive power coupling factor is proposed to qualitatively determine the coupling degree between each DC system, and then multiplied by the margin to guide the adaptive output of CFPREV. Reduce the CFPREV output when the margin of other DC systems is low to avoid the CF of other DC systems. Based on the CIGRE HVDC standard test model in the electromagnetic transient simulation platform PSCAD/EMTDC, the three-feed DC system was built as the simulation model, and the effectiveness of the proposed control method to suppress CF was verified.