Coordinated Control Method of Multi-infeed DC System for Commutation Failure Prevention

The mathematical relationship between reactive power consumed by DC system and AC voltage on inverter bus is derived under different control modes, respectively.Based on these relationships, a new definition of multi-infeed short circuit ratio (MISCR) is formulated from the power-flow model of PQ decoupled method.The specific influence of different control modes on voltage stability is analyzed.Considering the relationship between MISCR and voltage stability of AC/DC hybrid system, the characteristics of values of new MISCR are derived.Case study of an actual regional power grid validates these characteristics.

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).

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.

Commutation failure is one of the most common faults in the operation of high voltage direct current transmission (HVDC). This paper analyzes the influence of voltage amplitude reduction, zero-crossing displacement, and voltage waveform distortion on the commutation process based on the phenomenon of commutation failure in HVDC transmission system and its principles. In view of the distortion of the commutation voltage waveform caused by the low-order background harmonics, the DC system is more prone to commutation failure, and the simulation verification is carried out to provide a reference for the prevention of commutation failure.KeywordsHVDC transmissionCommutation failureInverterAC background harmonics

Switching on no-load transformer in AC system will cause serious inrush current. When the inrush current travels to the AC bus of inverter in DC system, it will lead to the variation of fundamental voltage and distortion, which inevitably affect the commutation process of the inverter. At present, the research mainly focuses on the variation of fundamental voltage, while the commutation failure caused by voltage distortion is rare. To ensure the safety and stability of the power grid operation, this paper elaborates the mechanism of commutation failure created by voltage distortion based on harmonic voltage-time area method, and also proposes an evaluation method which can quickly identify the commutation failure caused by inrush current. Finally, the PSCAD/EMTDC simulation results are provided to illustrate the effectiveness of the proposed method.

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.

Based on the simulation model of ± 800kV UHVDC transmission system established by PSCAD/EMTDC simulation software, the influence of different AC faults on DC system is studied. A single phase reclosing strategy is used in the DC system near the converter station. Aiming at this kind of situation has carried on the simulation, the simulation results show that the single-phase earth fault in the inverter system is easy to cause commutation failure. The simulation results are compared with the recorded files, and the waveforms of the two are consistent, which proves that the simulation results are credible.

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.

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 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.

Commutation failure is one of the common faults in LCC-HVDC transmission systems, threatening safety and stability of power grid. To focus on the key issues of commutation failure and reduce commutation failure frequency, a review of commutation failure in LCC-HVDC transmission systems is given in this paper. Firstly, the mechanism of commutation failure is analyzed from both device and system aspects. Since commutation failures are mostly caused by AC system faults on the inverter side, commutation failures are classified into three categories: first commutation failure, continuous commutation failure during the fault period and commutation failure after the fault. Then, the influencing factors and prevention methods of commutation failure in each category are summarized. Finally, the problems to be solved in the field of commutation failure are pointed out.

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.

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.

Introduction: The critical extinction angle of the HVDC converter is closely related to commutation failure and varies with the operating conditions. Existing research ignores the dynamic variation characteristics of the critical extinction angle, which will weaken the effect of the commutation failure suppression strategy. Considering the dynamic variation characteristics of the critical extinction angle, a commutation failure suppression strategy by dynamically compensating the extinction angle is proposed to improve the effect of commutation failure suppression. Methods: Firstly, this paper analyzes the influencing factors of the critical extinction angle, and establishes a calculation model of the critical extinction angle. Then, based on the proposed model, the influence of the critical extinction angle variation on the commutation failure process is analyzed. Considering the dynamic variation characteristics of the critical extinction angle during AC short-circuit fault, a commutation failure suppression strategy based on extinction angle compensation is proposed. Results: The proposed strategy can improve the suppression effect of commutation failure under different operating conditions, decrease the drop of AC voltage, and increase the recovery speed of DC power. Discussion: The proposed control strategy takes into account the dynamic variation of the critical extinction angle, enhances the suppression effect on commutation failure in HVDC systems, and improves the grid stability in modern power systems. Conclusion: Commutation failure is affected by the dynamic variation characteristics of the critical extinction angle. The proposed strategy considers the characteristics that can accelerate the fault recovery and improve the commutation failure resistance ability.

This letter analyzes the mechanism of the AC bus overvoltage at the LCC-HVDC rectifier station caused by the sending-end AC faults. Firstly, the influence of commutation failure on relevant electrical quantities is analyzed. After the commutation failure disappears, the inverter switches to constant current control. Under the joint action of the constant current control of the rectifier, the DC current and the trigger angle of the rectifier decrease rapidly, which reduces the reactive power absorbed by the rectifier. With the recovery of HVDC transmission capacity, the sending end AC bus voltage sharply rises, which increases the reactive power output of the filter. Reactive power redundancy occurs in the sending end AC bus leading to overvoltage. The suppression method proposed in this letter can effectively increase the reactive power absorbed by the LCC-HVDC rectifier during fault recovery. Besides, the method can also significantly enhance the minimum value of the inverter extinction angle and avoid possible commutation failure. The effectiveness and applicability of the strategy are confirmed through Electromagnetic Transient simulation.