Analysis of Inverter Commutation Failure Caused by Background Harmonics

Commutation failure is one of the commonest events in high voltage direct current (HVDC) systems. In the process of commutation failure, the DC voltage, current and power change drastically and impact the sending-side grid. At present, the related research lacks the quantitative analysis of transient overvoltage, which is caused by commutation failure in sending-side DC systems. Aiming at this problem, the equivalent model of the sending-side system is established. Considering characteristics of reactive power losses caused by the rectifier during commutation failure, analytical expression of transient voltage at the commutated bus is derived, then a quantitative calculation method of transient overvoltage and influencing factors are obtained. The results show that the weaker the AC system, the higher the transient voltage, and the transient over-voltage rises nonlinearly with the increase of the steady-state transmission capacity. Finally, the accuracy and effectiveness of the method are verified with the HVDC digital-physical hybrid simulation model based on Hypersim.

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.

Coordinated power control of electrochemical energy storage for mitigating subsequent commutation failures of HVDC

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.

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.

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 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).

Commutation failure (CF) is an inherent drawback of line-commutated converter high-voltage DC (LCC-HVDC) system. And CF prevention (CFPREV) control is widely applied in UHVDC lines to mitigate the subsequent CF. However, the formation of a multi-infeed UHVDC system induces power exchange and voltage interaction between AC buses during the transient process, making the behaviors and mitigation of CFs more complex. Consequently, exploring the impact of CFPREV control on adjacent stations and optimizing the CFPREV control to adapt to the interaction characteristics in a multi-infeed system is necessary. Considering the interaction characteristics, the impact of CFPREV control on the commutation process of inverters in a multi-infeed system is analyzed through formula derivation and simulation verification. It is found that although CFPREV control can effectively mitigate the CFs of the inverter near the fault, it will further increase the risk of concurrent CF (CCF) of remote inverters due to the interaction on the AC side. To solve this problem, a coordinated control scheme of CFPREV controls in a multi-infeed UHVDC system is proposed. The output of the CFPREV control of the inverter near the fault can be adjusted adaptively according to the commutation margins of remote inverters. And the probability of CCF caused by CFPREV control is consequently decreased. Case studies are conducted based on Henan provincial multi-infeed UHVDC system in China to verify the analysis result and the effectiveness of the coordinated control scheme of CFPREV controls.

This paper analyzes the essence of commutation failure of HVDC transmission systems, and points out that the load model has an important influence on commutation failure. The Commutation Failure Immunity Index (CFII) is used to quantitatively calculate the influence of the load type and percentage of the induction motor load on the commutation failure. It is proposed that the induction motor load can provide the rotary inertia to the system at the instant of fault, suppress the converter bus voltage drop and reduce the probability of commutation failure. The above discussion is verified 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.

Chinese energy resources and load are in reverse distribution, and it is necessary to vigorously develop UHV AC and DC technologies suitable for long-distance and large-power transmission. The pattern of UHV AC-DC hybrid power grid has been initially formed at present. However, the faults of UHV DC with large transmission power and complex fault forms have great impact on the power grid in both active and reactive power. In this paper, based on the analysis of the current situation and characteristics of UHV AC/DC power grids in China, the impact of UHV DC restart and commutation failure on the stability of the power grid is analyzed. The different types and triggering factors of DC commutation failures are analyzed, and the effects of single or multiple DC commutation failure types on the transient angle stability, the voltage stability and the transient overvoltage are simulated. The UHV DC restart fault time sequence is studied, and the effects of different restart modes on the system transient angle stability and transient overvoltage are demonstrated through simulation. The research results have laid the foundation for grasping the UHV DC fault morphology mechanism and the influence law on the stability of the power grid, and provided support for the safe and stable verification of the UHV AC/DC power grid.

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.

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.

Commutation failure (CF) is one of the common faults in High-voltage direct current (HVDC) transmission systems, and commutation failure prediction control (CFPREV) is a widely adopted engineering strategy to avoid the initial cf However, there are still problems such as delayed fault response and inflexible parameter adjustment, which affect the effectiveness of CF suppression. Therefore, an improved commutation failure prevention control strategy is proposed. This strategy uses the expected voltage drop area as an index to quantitatively evaluate the risk of commutation failure. Meanwhile, because this index responds slowly during the early stage of a fault, a pre-startup mechanism based on fault accumulation is introduced. It provides dynamic compensation for the expected voltage drop area index and accelerates its response. Finally, an analytical method for calculating the firing angle adjustment is derived using the commutation area theory. The proposed strategy achieves closed-loop optimization from ‘fault detection’ to ‘quantitative adjustment,’ thereby enhancing the system’s adaptability and suppression capability against cf Simulation results demonstrate that, compared with the conventional CFPREV, the proposed strategy can respond to faults significantly earlier, achieving a greater increase in the extinction angle. Across multiple fault test cases, the CF probability of the conventional CFPREV is approximately 58%, whereas the proposed strategy reduces it to 32%, substantially enhancing the system’s ability to suppress cf

The electromagnetic transient can be used to analyze the stability of power systems. In the paper, the electromagnetic transient of an AC-DC interconnected system under different faults is investigated, and interaction between the AC system and DC system is analyzed. Firstly electromagnetic transient models for the components in a high voltage direct current (HVDC) transmission system are presented, including model for converter transformer, harmonic filters, transmission line and control system. Then simulation based on electromagnetic transient model for a ± 800 kV bipolar HVDC-AC interconnected system is carried out, and both the steady state and step response performance are checked to verify the accuracy of the model. Finally electromagnetic transient of the system under different faults is investigated, including different grounding faults on AC side, fault on DC side and fault of filters. Transient responses are analyzed, including the recovering time from oscillation caused by faults and possibility of commutation failure (CF).