Research on UHV DC Fault Form and Its Influence

The UHVDC sending-end power grid with high proportion of renewable energy has serious transient voltage stability problems. Firstly, the structure of renewable power systems and the overvoltage protection logic of the convertor-based renewable generators are introduced. Then, the transient overvoltage characteristic of the high renewable power-penetrated UHVDC sending-end power grid under the DC blocking fault and the commutation failure is analyzed. And the mechanism of renewable generator cascading overvoltage tripping is analyzed in the meantime. On this basis, the transient voltage security index system based on the multiple-two-element table, consisting of the local index for a single node and the global index for the whole system, is established under the single fault and the multiple-fault set respectively. Finally, the effectiveness of the proposed indexes is shown by some case studies of Qinghai Power Grid in China.

With the rapid development of the Chinese power grid, its dynamic behavior becomes increasingly complex. How to prevent voltage instability and voltage collapse at the receiving-end system has become an urgent problem at present. To prevent voltage collapse, the key is to determine the stability margin at the operating point of the power system, and the load margin is the most basic and widely accepted voltage stability margin indicator. Considering the role of medium and long-term slow-acting elements in the actual system that affect the system voltage stability, this paper combines the static voltage stability analysis with the transient, medium, and long-term dynamic voltage stability analysis based on whole process simulation, and uses time-domain simulation methods to evaluate the voltage stability of the actual system. By analyzing the various dynamic characteristics of generator overexcitation limits and transformer on-load regulation on the development process of voltage-collapsing accidents, this paper proposes an evaluation index for the whole process dynamic stability margin of the receiving-end system. On this basis, the whole process dynamic defense system of the power grid is also constructed. The proposed method is validated by using a provincial power grid in China as an example.

With the large-scale deployment of renewable energy, transmission systems for new energy sources are increasingly exposed to transient overvoltage issues induced by DC blockages and commutation failures. To address the challenges of an imprecise response to multiple fault scenarios and the inefficiency of independent device actions in existing dynamic reactive power control schemes, this paper proposes a coordinated optimization control strategy integrating multiple dynamic reactive power sources tailored to different fault characteristics. An equivalent model of the renewable energy DC transmission system is established to elucidate the underlying mechanisms of transient overvoltage formation under various fault conditions. By employing trajectory sensitivity analysis and parameter perturbation methods, the influence patterns of control parameters on transient overvoltage behaviors across different fault scenarios are quantitatively assessed, thereby overcoming the limitations of traditional empirical parameter tuning approaches. Subsequently, a multi-source coordinated optimization model is developed with the objective of minimizing transient overvoltages under simultaneous dual-fault conditions. A multi-objective particle swarm optimization algorithm, incorporating comprehensive trajectory sensitivity and dynamically adaptive inertia weights, is introduced, alongside Pareto front theory, to achieve rapid and balanced optimization among competing objectives. Simulation results validate that the proposed strategy significantly enhances transient overvoltage suppression across diverse fault conditions. The findings provide robust theoretical foundations and practical guidance for the refined parameter tuning and high-efficiency coordinated control of dynamic reactive power sources in renewable energy transmission systems.

Offshorewind farms inherently need alternating current (AC) grid connection withundersea cables. However, this requires reactive power compensation, which iscostly and not practical. Moreover, it threads the stability of the system. Forthese reasons, high voltage direct current (HVDC) transmission is a promisingsolution since no reactive power is needed. Furthermore, the DC power flowbetween offshore and onshore locations as well as the reactive power at eitherside can easily be controlled by voltage source converters (VSCs). In thispaper, the modeling and control of an offshore wind farm connected to themainland grid power with HVDC transmission is proposed. A control strategy isproposed to control active and reactive power at the offshore wind farmconverter and DC voltage at the onshore converter. The overall system,including wind farm, offshore and onshore converters, HVDC transmission, and ACgrid side, were tested and verified in a simulation environment. The designedcontrol systems of the VSCs were tested by various case studies.

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.

Direct Current (DC) transmission system is an important part of the interregional grid. However, the frequency impact of power grid caused by DC blocking is more serious with the continuous increase of scale of single DC transmission. The frequency coordination control system is applied to the receiving-end power grid in High Voltage Direct Current (HVDC) transmission system. In the cases of single-infeed HVDC blocking, multi-infeed HVDC blocking or fast power drop, the frequency coordination control system can reduce the frequency fluctuation of power grid by instantly promoting the transmission power of the rest normal DC systems and reducing power loss. The frequency coordinated control substation is an executive device of the system at the converter station. It keeps communicating with the coordinated control master station for real-time monitoring the operation of converter station, reports the fault and executes the remote power boost instructions. Taking the frequency coordinated control substation of 500 kV conventional DC systems as a research background, the load of converter station in a regional power grid is analyzed. The working mechanism of frequency coordination control system in the cases of current converter station fault or other station faults is analyzed. The actual effect of a frequency coordination control system on a converter station in East China power grid is investigated by testing the actual power, voltage and current variation of a conventional DC transmission system.

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.

This paper investigates the influence of a high voltage direct current (HVDC) overlay grid on the transient voltage and rotor angle stability with respect to HVDC converter fault ride-through behavior. For this purpose, the studied meshed hybrid AC/DC transmission grid is established in MATLAB/Simulink with user defined dynamic models of voltage-source converters (VSC) considering fault ride-through requirements of the German Technical Connection Rules for extra high-voltage (VDE-AR-N 4130). The voltage profile during three-phase short-circuit faults as well as bus voltage relative entropy after fault-clearing as stability indices are analyzed. The influence of k-factor and power system stabilisers on transient stability is also investigated.

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.

The failure of high-power DC commutation leads to large fluctuations in the voltage of the transmitting end grid, which brings great harm to the safe and stable operation of the transmitting end grid. In this paper, the detailed analysis of PMU data is carried out for the voltage fluctuation events of the transmitting end caused by the frequent DC commutation failure in recent years. By constructing a simulation model in BPA and RTDS, the fault back-tracking of the voltage fluctuation of the power supply with the commutation failure is carried out. The simulation results show that the main cause of the fluctuation is caused by the timing coordination and the delay of the rectification side AC filter. This is of great significance for reducing voltage fluctuations in the case of subsequent DC system commutation failure, and it also has great significance for maintaining safe and stable operation of the power grid.

Performance and reliability of electrical power grids under cascading failures

Since a large-scale wind farm is connected, it has an impact on the hybrid High Voltage Alternating Current/Direct Current (HV AC/DC) power grid with the characteristics of 'strong DC and weak AC'. When the connected capacity of the wind farm drops instantaneously, it may cause the DC system commutation failure. In this paper, firstly, the impact of wind power grid connection on the converter bus voltage is analyzed. Then, the relationship between the commutation bus voltage variation and the critical commutation voltage reduction is established, the correlation factor of the DC commutation failure caused by the wind power grid connection is proposed, and the criterion for determining the DC commutation failure caused by the wind power grid connection is defined. As a result, the maximum variation of wind power access capacity could be obtained.

Power grids with multi-DC high voltage direct current (HVDC) commonly face challenges such as inadequate voltage support, exacerbated interactions between AC and DC, and complex and variable operation modes. Severe AC faults in these grids can easily trigger consecutive failures in DC commutation, potentially leading to a collapse in the voltage across the entire network. This study introduces a synchronized optimization technique aimed at improving voltage stability within receiving-end power systems by adjusting the parameters of the Voltage Dependent Current Order Limiter (VDCOL). Firstly, principles for adjusting VDCOL parameters are proposed grounded on voltage instability mode. Derived from the sorting outcomes of the multi-infeed short-circuit ratio, parameters of VDCOL are adjusted to ultimately suppress commutation failures and enhance the capability of receiving-end power grids to accept external power. Finally, the simulation result verifies that optimizing VDCOL parameters according to the proposed method significantly increases the capability of receiving-end power grids to accept external power.

With the UHV (Ultra High Voltage) power grid construction and the interconnection of regional power grids, the scale of power grids in China is increasing rapidly. At the same time, significant uncertainty and variability is being introduced into power grid operation with the integration of large-scale renewable energy in power systems. All of these pose an enormous challenge to the operation control of power systems in China. For a long time, online static security analysis, as an important part of EMS (Energy Management System), has been an effective tool for power grid operation. However, it is increasingly difficult for traditional static security analysis in serial computing mode to be online applied in bulk power grids in China. A new practical parallel approach for online static security analysis is put forward in this paper. A multithread parallelism is introduced into contingency screening, detailed contingency evaluation and decision support for reducing the execution time. By employing the multithread technology, the hardware resources of multi-processor/multi-core computer can be fully used and the program can be speeded up effectively. The performance of the parallel static security analysis is demonstrated by tests on two large-scale power systems. The test results show that the proposed method can be online applied in real bulk power grids.

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.