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

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.

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.

Line-commutated converter based high voltage direct current (LCC-HVDC) transmissions are widely applied in delivering large-capacity electricity in China. Multiple large-capacity LCC-HVDC transmissions strengthen dynamic coupling between AC and DC networks. Existing literature has reported some stability problems caused by dynamic interactions between AC and DC power grids, however, the mechanism behind these problems has not been totally understood and revealed. To analyze dynamic interactions between AC and DC power grids, this article presents a small-signal model to understand the external characteristics of LCC-HVDC stations based on the motion equation concept. Comparison of time-domain simulation response with the detailed switching model shows that the proposed model can hold main behaviors of concern. Dynamic interactions between AC and DC power grids are studied based on the proposed model in multi-infeed LCC-HVDC systems. Analytical results demonstrate that there exists power dynamics in receiving AC grid when sending AC grid subjects to a disturbance, and effects of converter stations’ controller parameters on power dynamics are explained by the physical relationship among state variables of the proposed model.

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.

Large-scale Voltage Source Converter based High Voltage Direct Current Transmission, (VSC-HVDC) and line-commutated converter based high voltage direct current (LCC-HVDC) external power receiving-end power grids require the AC system to provide a considerable degree of voltage support capability to ensure the static voltage stability margin of the system. There is a coupling relationship between equipment and equipment, and between equipment and AC network. How to accurately quantify and evaluate the strength of the receiving-end power grid and the voltage stability margin has become an urgent problem in the power industry. Based on this, the calculation method of the VSC-HVDC and LCC-HVDC hybrid double-feed system equivalently approximated to the single-feed LCC-HVDC is summarized based on this. The defects of this method are analyzed, and a new method is proposed from the perspective of considering the external characteristics of equipment control. Quantification method of voltage support strength, and analyze and compare the difference between the two approximate calculations. Finally, a simulation example is used to compare and analyze the accuracy and effectiveness of the proposed methods.

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

The commutation failure of high-voltage direct current (HVDC) transmission system will have a huge power impact on the AC system. It is necessary to accurately identify the commutation failure of the HVDC for the security and stability control device (stability control device). At present, the stability control device which has already been applied in engineering for HVDC identifies the commutation failure of HVDC depends on the command of DC control and protection system, and there is a certain risk. The valve side current of converter transformer and the DC current of valve group that can be directly collected by the engineering are studied. Starting from the self-demand of the stability control device, new HVDC commutation failure and continuous commutation failure criteria are proposed based on the original commutation failure protection principle. Also, RTDS test verification was carried out.

The continuous commutation failure (CCF) of high-voltage direct-current (HVDC) transmission system will cause consecutive impacts to the AC-DC hybrid power system. This paper proposes an improved firing angle control strategy with the DC power recovery rate and AC fault severity considered to suppress CCF. The dominant factors and mechanism of CCF are analyzed. Then, the method of dynamically adjusting firing angle according to the DC power recovery and AC voltage drop is introduced. During the fault recovery after first commutation failure, firing angle of the inverter side can be adaptively reduced to improve commutation margin. Case studies based on the CIGRE benchmark model validate the effectiveness of the proposed control strategy.

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.

Accurate detection and effective control strategy of commutation failure (CF) of high voltage direct current (HVDC) are of great significance for keeping the safe and stable operations of the hybrid power grid. At first, a novel detection method for consecutive CF is proposed. Concretely, the 2nd and higher orders’ derivative values of direct current are summarized as the core to judge CF by analyzing the physical characteristics of the direct current waveform of the converter station in CF. Then, the Daubechies wavelet coefficient that can represent the 2nd and higher order derivative values of direct current is derived. Once the wavelet coefficients of the sampling points are detected to exceed the threshold, the occurrence of CF is confirmed. Furthermore, by instantly increasing advanced firing angle β in the inverter side, an additional emergency control strategy to prevent subsequent CF is proposed. Eventually, with simulations of the benchmark model, the effectiveness and superiorities of the proposed detection method and additional control strategy in accuracy and rapidity are verified.

Control strategy improvement is an effective method for commutation failure mitigation in line-commutated-converter-based high voltage direct current (LCC-HVDC). However, for the multi-infeed HVDC system with multiple dc links located in close mutual proximity, existing strategies based on adjustment of dc current or the firing angle may lead to negative influence even commutation failure in adjacent HVDC. Therefore, in order to achieve coordinated recovery of multi-infeed HVDC systems, novel control strategy for improvement of the voltage dependent dc current order limiter (VDCOL) is put forward after analyzing the reactive power interaction between dc systems. Then crucial parameters in the proposed strategy are determined based on evaluation of influence on stability of the multi-infeed HVDC system caused by each dc system and reactive power-voltage sensitivity between ac busbar of inverters. Simulation results in PSCAD/EMTDC verify effectiveness and advantage of the method on commutation failure mitigation as well as coordinated recovery performance.

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.

In a high-voltage direct current (HVDC) transmission system, commutation failure at the receiving end may lead to transient overvoltage at the sending end converter bus of the weak alterative current (AC) system. Firstly, the principle calculation method of overvoltage generation at the sending end after commutation failure is analyzed. Combined with the output characteristics of the hydroelectric excitation system, a coordinated control strategy for hydroelectric and DC systems is proposed. Since the voltage and current values at the DC outlet of the rectifier side change first after a fault occurs at the receiving end, the relationship equation between DC voltage and AC bus voltage is derived and it is used as an input signal to construct additional excitation control for hydropower stations. The proposed strategy is verified by establishing a simulation hydrogen–wind–solar model bundled via a DC sending system in PSCAD/EMTDC. The simulation results illustrate that the transient overvoltage suppression rates are all more than 35%, and the maximum is 38.53%. The proposed strategy can reduce the overvoltage by 0.126 p.u. compared with the International Council on Large Electric Systems (CIGRE) standard control strategy.

Small-signal instability and supplementary coordinated damping-control of LCC-HVDC system with STATCOM under weak AC grid conditions

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.