Commutation Failure Research Articles

Impact of Strength and Proximity of Receiving AC Systems on Cascaded LCC-MMC Hybrid HVDC System

The cascaded hybrid LCC-MMC inverter is considered as a feasible option to mitigate the commutation failure problem of conventional LCC-HVDC while maintaining bulk power transmission capability. However, the coupling between different types of converters may result in instability when the inverter integrates with weak AC systems. This paper establishes a small-signal model of the cascaded LCC-MMC inverter based hybrid HVDC system and validates the model against PSCAD/EMTDC. Through eigenvalue analysis, the impact of the strength and proximity of receiving AC systems on the small-signal stability is investigated, and the stable zone of the strength of receiving AC systems is identified. Besides, a supplementary coupling mitigation control is proposed to tackle the instability problem under weak AC systems integration. Both analytical results and time-domain simulations demonstrate the validity of the supplementary control.

Novel travelling wave fault location principle for VSC-HVDC transmission line

Currently, voltage source converter (VSC) based high voltage direct current (HVDC) technology is widely utilized in power transmission system. It has high transmission efficiency, no commutation failure issue and ability to supply power to passive system. In order to enhance the reliability of VSC-HVDC transmission system and accelerate the maintenance progress, travelling wave (TW) fault location principle is adopted to discriminate the exact location of transmission line (TL) fault, and its fault location accuracy is highly related to TW arrival time detection and TW propagation velocity. Therefore, aiming to enhance the fault location accuracy, the paper proposes a novel TW fault location principle for VSC-HVDC transmission line. Firstly, the detailed calculation method of propagation velocity of TW along HVDC-TL is described. Secondly, a novel TW fault location principle is proposed including TW arrival time detection method and TW’s accurate frequency calculation method. Eventually, based on PSCAD/EMTDC, a typical ±400kV VSC-HVDC simulation model is established to validate the robustness of novel TW fault location principle against fault type issue, fault resistance issue and fault location issue.

Rapid Recovery Control Method Based on Improved VDCOLs for Hybrid Multi-Infeed DC Transmission System After AC Failure

Continuous commutation failure is very likely to occur in the hybrid Multi-infeed high-voltage direct current (HMIDC) after AC failure. In order to improve the recovery quality after HMIDC failure, an AC-DC voltage-dependent current order limiter (VDCOL) based on system strength index is proposed in this article. Firstly, the control mode transition process and system recovery process after DC failure are analyzed based on the hybrid multi-infeed DC transmission port model. Then, considering the impact of AC voltage and DC voltage input signals of VDCOL on AC voltage recovery and DC power recovery, respectively, the interaction factor and strength index of the hybrid multi-infeed system are constructed. Moreover, the weight coefficient of AC and DC voltage is calculated according to the strength of the multi-infeed system. Finally, a three-infeed hybrid DC transmission simulation model is built in the MATLAB/Simulink digital simulation platform. The simulation results demonstrate that the rapid recovery strategy proposed in this article can effectively suppress continuous commutation failure and improve the recovery speed of AC voltage and DC power.

Study on Dynamic Characteristics of UHVDC System under Hierarchical Infeed Mode with STATCOM

This paper focuses on the investigation of the dynamic characteristics of Ultra High Voltage Direct Current (UHVDC) system Under Hierarchical Infeed Mode (UHVDC-HIM) with Static Synchronous Compensator (STATCOM). The STATCOM at the inverter side of UHVDC-HIM is developed in PSCAD/EMTDC, and their power flow equations are derived. Based on these equations, the control schemes for rectifier and inverter side of UHVDC-HIM and STATCOM are properly designed. Several dynamic characteristics indices such as Commutation Failure Immunity Index (CFII), Commutation Failure Probability Index (CFPI), fault recovery time and Temporary Overvoltage (TOV) are evaluated in detail. The impact of different sizes of STATCOM and Short Circuit Ratio (SCR) of AC sources on dynamic characteristics of UHVDC-HIM system during single-phase and three-phase faults are examined. The analysis shows that higher SCR values and greater capacity of STATCOM can make the UHVDC-HIM system less vulnerable to Commutation Failure (CF), considerably enhance the fault recovery time and effectively reduce the TOV.

A control strategy of converters based on constant extinction area for UHVDC system under hierarchical connection

The Ultra High Voltage Direct Current (UHVDC) systems under hierarchical connection, while improving the voltage support and current assimilative capacity of the receiving-end system, also bring new challenges to the security and stability of the power grid. An AC short-circuit fault at the receiving-end system can easily induce commutation failures (CFs) of the high-end and low-end converter at the same time, which seriously affects the stability of the system. Therefore, it is urgent to study the coordinated control strategy for the commutation failures of UHVDC systems under hierarchical connection. This paper analyzes the coupling mechanism and CFs characteristics of the high-end and low-end converters of a UHVDC system under hierarchical connection. Based on the commutation area theory of the three-phase full-wave bridge circuit, a coordinated control strategy is proposed to suppress CFs of the non-faulty layer converter of the UHVDC system under hierarchical connection. It can reserve enough extinction area for the non-faulty layer converter after an AC system failure occurs, thus preventing the occurrence of commutation failure. Finally, based on PSCAD/EMTDC, Zhundong-Wannan ±1100 kV UHVDC was built and the effectiveness of the proposed control strategy was simulated and verified. The simulation results show that the proposed control strategy can effectively suppress the simultaneous commutation failure of inverter station converters when symmetrical faults and asymmetrical faults of different levels occur in the receiving-end grid. This has important engineering significance for improving the stability of the DC transmission system.

Coordinated DC voltage control strategy of hybrid HVDC with multi‐infeed MMC inverters considering commutation failures

Coordinated DC voltage control strategy of hybrid HVDC with multi‐infeed MMC inverters considering commutation failures

Overvoltage Suppression Strategy for Sending AC Grid With High Penetration of Wind Power in the LCC-HVdc System Under Commutation Failure

With the increase of wind power installation capacity, the line commutated converter (LCC)-based high-voltage direct current (HVdc) transmission system is an effective scheme for long distance transmission of wind power. Commutation failure is a common fault for LCC-HVdc transmission system. During the commutation failure, the voltage of sending ac grid presents the “first decreased then increased” characteristic. In order to avoid the overvoltage under the commutation failure causing the wind power generation system (WPGS) to separate from the power grid, it is necessary to suppress the overvoltage of sending ac grid under commutation failure. This article quantitatively analyzes for the first time the transient voltage characteristics of the sending ac grid under commutation failure. The analysis indicates that the delay of the grid voltage detection part in the wind turbine is the significant influence factor on the overvoltage of sending ac grid. Based on the analysis results, this article proposes the delay compensation control strategy of WPGS to suppress the over voltage of the sending ac grid with high penetration of wind power under commutation failure. The effectiveness of the strategy is verified by simulation and control hardware in-loop experiment, and it is also verified that the strategy is also applicable for conventional low-voltage ride through faults, which shows that the control strategy is universal.

Reactive Power Control Strategy for Inhibiting Transient Overvoltage Caused by Commutation Failure

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).

Analytical Expression on Transient Overvoltage Peak Value of Converter Bus Caused by DC Faults

This letter proposes an analytical expression on transient overvoltage peak value caused by DC blocking (DCB) and commutation failure (CF). The proposed method can obtain the transient overvoltage risk without modeling the complex AC/DC hybrid system. The method only requires the equivalent parameters of the AC system and the operation parameters of the DC system to effectively calculate the overvoltage without using the dynamic simulation software. The simulation results verify the effectiveness and correctness of the proposed method.

Quantifying the Contribution of Dynamic Reactive Power Compensators on System Strength at LCC-HVdc Converter Terminals

This paper presents improved indices to evaluate the contribution of reactive power compensators at HVdc converter terminals. The presence of such compensators impacts converter performance, such as the maximum available power (MAP) limit, susceptance to commutation failure, fault recovery time and temporary over voltage. Traditionally, the Effective Short Circuit Ratio (ESCR) has been used to indicate system strength. However, this index cannot be directly applied when power-electronics based converters are used to provide the reactive power support. The approach of this paper is to use an improved form of the ‘Apparent increase in Short Circuit Ratio’ (AISCR) index to quantify the impact of the compensator on the HVdc system. This index is evaluated by comparing the performance with the reactive power device with a system without the device, but having the same performance. The paper shows that a single AISCR index cannot quantify all behaviors and suggest modifications to AISCR for each of several HVdc phenomena. In this paper two types of compensators are considered – the Synchronous Compensator and the Static Synchronous Compensator (STATCOM).

The influence of open-phase operation of inverter side AC subsystem on commutation process and countermeasures

Commutation failure can lead to DC power fluctuation or even DC pole blocking, threatening the safety and stability of power system. As an abnormal operation state, open-phase operation of inverter side AC subsystem (OPOISAS) occurs at times and affects commutation process. In this paper, the influence of OPOISAS on commutation process is analyzed first. And the analysis shows that the larger the negative-sequence voltage amplitude (NSVA) of the commutation bus is, the greater the risk of commutation failure will be. To deal with the risk of commutation failure, the formula of the negative-sequence voltage of commutation bus during OPOISAS is deduced. Based on the formula, the law of NSVA variation with different factors is analyzed. Meanwhile, several countermeasures are given from the perspective of system planning and operation to reduce the risk of commutation failure during OPOISAS. Simulation results demonstrate that the analysis is correct and the countermeasures are effective.

AC system’s strength evaluation of UHVDC system under hierarchical connection mode

The inverter’s end of ultra high voltage DC (UHVDC) system under hierarchical connection mode (HCM) is connected to 500 and 1000 kV AC buses through high end converter and low end converter transformers, respectively. The two AC buses are interconnected via coupling transformer and the tie line. This complex structure of interconnection at inverter’s end can make the understanding unclear about each AC system’s strength of UHVDC-HCM. Therefore, this paper covers the system’s strength evaluation by developing the “Hierarchical mode effective short circuit ratio” (HMESCR) index. The HMESCR incorporates the impact of neighborhood AC networks, impedance of coupling transformer, the tie line and strength of the objective AC network. Based on the proposed index, an equivalent model for UHVDC-HCM system is developed. An equivalent model is verified under the steady state conditions such as maximum available power and the transient conditions such as commutation failure immunity index under AC faults. In addition, three different cases are considered in order to get the in-depth and comparative performance analysis of an equivalent model with UHVDC-HCM system. MATLAB and PSCAD/EMTDC are used as analysis tools for illustrating the steady state and transient state conditions. The analysis indicates that the presented equivalent model has similar behavior as UHVDC-HCM system under both steady state and transient state conditions, which verifies the accuracy of proposed index in assessing the strength of both AC systems of UHVDC-HCM.

Reactive power compensation optimization configuration method to reduce the risk of multi-circuit DC commutation failure

Aiming at the risk of simultaneous commutation failure of multiple DCs in multi-infeed DC areas, a reactive power compensation optimization configuration method to reduce the risk of multiple DC commutation failures is proposed, and the type, location and capacity of reactive power compensation devices are configured. Firstly, the type of reactive power compensation devices is determined by comparing SVC and STATCOM simulation; Secondly, the weak voltage area through typical faults in multi-infeed DC areas is found, and then the best reactive power compensation site is determined according to the concept of node relative transient voltage drop area RTVDAI index; The aim is to reduce the effect index of commutation failure risk and project economy, the best installation plan is determined by controlling the investment cost and reducing the risk of commutation failure. The closeness in the entropy method is used to evaluate the advantages and disadvantages of each capacity configuration plan, and then CFTOOL toolbox in MATLAB is used to fit the comprehensive analysis index of nearness. The optimal installation capacity is determined by analyzing the fitting curve of the closeness comprehensive analysis index. Finally, the engineering applicability of the configuration scheme was verified through simulation.

A Dynamic Series Voltage Compensator for the Mitigation of LCC-HVDC Commutation Failure

To effectively mitigate the commutation failure (CF) of the line-commutated converter high-voltage direct current (LCC-HVDC) transmission system, a dynamic series voltage compensator (DSVC) scheme is proposed. The DSVC consists of three integrated gate-commutated thyristor (IGCT)-based full-bridge submodule (FBSM) chains to match the bulk power transmission of the LCC-HVDC. Inserted between the transformer and the AC port of the inverter, the DSVC assists the commutation process by superposing the transient voltage to the AC line voltage and increasing the commutation margin of the inverter valve. Operation and control strategies, which are two critical techniques for the DSVC, are designed based on the working states of the FBSMs, assisting the commutation process and balancing the DSVC capacitor dynamic voltages. The CF detection method is another critical technique of the DSVC control and is improved considering the reverse time of the LCC-HVDC thyristors. Compared with the topologies proposed in existing literature, the DSVC has better performances in safe and stable operation due to the urgent control strategy considering situations beyond its ability. A detailed EMT simulation shows that the DSVC can respond quickly to mitigate CFs and validate the effectiveness of the overall design and key techniques. Furthermore, the operation and control strategy can limit the current stress once it is beyond the capability of the DSVC, which guarantees its feasibility and practicability.

Fault current limiting method based on virtual impedance for hybrid high‐voltage direct current with cascaded MMC inverters

The hybrid high-voltage direct current (HVDC) with cascaded multi-infeed MMC inverters is proposed in recent years, and it will be put into reality in near future. But the commutation failure of the line-commutated converter (LCC) inverter will make the whole HVDC system to be blocked because the DC fault current could reach beyond the safety region of the MMC inverter. To solve such problem, this letter proposes a novel fault current limiting method based on the LCC control strategy, namely, the virtual impedance method. Such method can limit the fault current effectively and need no extra devices, which can avoid the block of the whole HVDC system on one side, and save the cost of the construction on the other side. The theoretical analysis shows that the proposed method can improve the high-frequency characteristic of the DC system and realise the fault current limitation effect. The simulations validate the analysis finally.

Line Commutated Converter Response during Total and Partial De-Blocking of a Bipolar MTDC System

This paper focuses on the fault blocking analysis and operational issues associated with MTDC systems incorporated in an AC network. The dynamic modelling of a line-commutated converter based bipolar multi-terminal direct current (LCC MTDC) system are shown, and the dynamic response of the converter during a DC converter fault is discussed. The converter controller design for both rectifiers and the inverters system was modelled for a realistic active power and extinction angle (γ) control with consideration to the VI characteristics of all the converter stations. An overall power controller was modelled for both converter pole. Two operational scenarios of converter fault were simulated using PSCAD EMTDC. The converter firing angle and extinction angle, as well as the voltage-dependent current order limiter, was monitored and plotted on a graph. Results show that the MTDC link became unstable during the full deblocking stage with a continuous occurrence of commutation failure. Furthermore, the results presented in this paper show that during partial converter de-blocking showed a favourable performance, as the power system remains stable and commutation failure of the MTDC system is prevented.

Quantitative Assessment for Commutation Security Based on Extinction Angle Trajectory

In order to reduce the risk of commutation failure (CF) in the AC/DC hybrid power system, the quantitative analysis on CF is required for on-line assessment and optimal control. This paper presents an accurate and reliable method to quantify the commutation security based on the trajectory due to the complexity of the high-voltage direct current (HVDC) model. Firstly, the characteristics of the extinction angle trajectory are analyzed under both commutation success and failure conditions. The commutation security margin index (CSMI) is then proposed for the HVDC systems. Moreover, a search strategy for parameter limits is put forward based on the sensitivity analysis of CSMI to accelerate the search speed with a guaranteed accuracy level. A modified IEEE 39-bus power system and an actual large-scale power system with 46 generators and 821 buses are utilized to verify the validity and robustness of the proposed index and strategy.

An Evaluation Method for Commutation Failure Caused by Waveform Distortion

Commutation failure is a key issue that affects the safe operation of HVDC transmission systems. Scholars at home and abroad have conducted research on the mechanism, evaluation indicators, and control strategies of commutation failure. However, most of the researches are aimed at the working conditions of commutation failure caused by a short-circuit fault on the AC side. There are few studies on the commutation failure caused by the harmonic injection on the AC side and the distortion of the AC bus voltage waveform. This paper clarifies the mechanism of commutation failure caused by waveform distortion, and proposes an index of harmonic commutation margin. On this basis, an evaluation method for commutation failure caused by waveform distortion is proposed. By predicting the commutation voltage-time area and optimization methods to assess whether the current voltage conditions have the risk of commutation failure. Finally, an electromagnetic transient simulation model is established on PSCAD/EMTDC, and the evaluation method in this chapter is verified by simulating the working conditions of the inverter side bus voltage waveform distortion. The results show its effectiveness in evaluating the commutation failure. The research provides a theoretical basis to the implementation of the control function and has broad engineering application prospects.

Influence of coupling characteristics on commutation failure of new hybrid HVDC transmission system

Hybrid cascaded multi-terminal DC transmission provides a more flexible transmission method, improves the voltage stability of the inverter AC system, and reduces the probability of commutation failure. The hybrid LCC-VSC at the receiving end of the DC system is a new type of topology.In order to study the influence of coupling characteristics between VSC and LCC on commutation failure in AC power grid at receiving end,firstly,a hybrid LCC-VSC simulation model is built on PSCAD to analyze the operation characteristics of the system during the fault and verify the reactive power support ability of VSC to LCC, and the improvement effect of VSC on LCC-HVDC under different reactive power control modes is compared and analyzed. In order to study the influence of AC/DC coupling on commutation failure of LCC, commutation failure immune factor (CFII) was introduced to study the influence of AC system strength on commutation failure of LCC. Finally, considering that the influence of AC/DC coupling on commutation failure is closely related to the degree of coupling, the electrical distance between LCC and VSC is further studied. The results show that the conclusion can provide theoretical guidance for the design and analysis of related projects.

Transient Voltage Control of Sending-End Wind Farm Using a Synchronous Condenser Under Commutation Failure of HVDC Transmission System

When the large-scale wind power is sent out through the high voltage direct current (HVDC) transmission system and a DC commutation failure occurs, the voltage of AC bus at the sending end decreases first and then increases. Suppose the reactive power supported in the low voltage ride-through process by various reactive resources is not timely returned. In that case, it may aggravate the voltage rise caused by the commutation failure, and the off-grid risk of wind turbine under high-voltage will be aggravated. In order to reduce the off-grid risk of wind turbines caused by the DC commutation failure, a transient voltage control strategy of DC sending-end regulator based on the online sequential extreme learning machine (OS-ELM) voltage prediction model is proposed. Firstly, the influence factors of commutation failures are analyzed. Aiming at the key factors, the real-time voltage comprehensive prediction model based on OS-ELM is used to predict the voltage increase during the commutation failure process and uses the voltage prediction results to optimize the transient response of the synchronous condenser. A large-scale wind farm together with the HVDC system is established in PSCAD to verify the effectiveness of the proposed scheme. Simulation results show that the proposed scheme can reduce the risk of wind power off-grid risk under DC commutation failures and increase the speed of voltage recovery at the point of common coupling.