A Discrimination Method for Cascading Commutation Failures in Multi-Infeed HVDC Systems within AC/DC Hybrid Grids

With more power electronics deployed in power system, the propagation of cascading failure become more complex and it is necessary to investigate the mechanism of cascading failure in the novel power system. This paper reveals a new cascading failure mode in multi-infeed high voltage direct current (HVDC) systems. Firstly, the mechanism of multi-timescale cascading failure is proposed. Then the simulation results of multi-infeed HVDC systems which is carried out in Power system department-Bonneville power administration (PSD-BPA) indicate that the dominant cascading failure modes in different timescales are different and they are coupling with each other. Finally, several dominant factors of different timescale cascading failure modes are pointed out.

A commutation failure risk analysis method considering the interaction of inverter stations

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.

Sensitivity analysis of commutation failure in multi-infeed HVDC systems: Exploring the impact of ac system representations and fault types

Summary form only given. The commutation failure vulnerability of multi-infeed HVDC converters to ac side faults is analyzed. Faults in multi-infeed systems can cause commutation failure events in the nearby local converter or concurrently in both converters. The immunity of the system to both these events is investigated and suitably parameterized. “Commutation Failure Immunity Index” (CFII) is calculated for multi-infeed systems with different ratings. Anomalous failure in valve current commutation due to voltage distortion has been investigated.

Commutation failure theory of HVDC system is proposed and its reason is analyzed. Faults at inverter bus in multi-infeed HVDC system can cause local commutation failure and even concurrent commutation failure. Anomalous commutation failure is simulated with six kinds of ac system strength, and it is more and more serious at first as the system strength increases then become less serious as the strength becomes stronger. Coupling impedance is changed to get its relationship with anomalous commutation failure.

Set up a model of typical minimum ac/dc system. From the electrical distance and the phase difference of steady state between fault point and converter bus to research the spatial characteristics of HVDC commutation failure. Simulation results show that under the condition of invariable system support strength, the electrical distance between fault point and the converter bus is the main factor affecting the spatial characteristics of HVDC commutation failure. The closer the electrical distance is, the more likely to cause a dc commutation failure. The influence of power flow to HVDC commutation failure is mainly embodied in the phase difference of steady state between the fault point and the converter bus. The lager the phase of the fault point is, relative to the phase of the dc converter bus, the easier the HVDC occurs commutation failure.

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.

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.

Recent years have witnessed a surge of extensive study upon the robustness of a system consisting of interdependent networks. Both the mechanism of cascading failures and the corresponding recovery strategies, which means to heal malfunction of a system before it runs completely into collapse, are two key issues in the field of robustness analysis of interdependent networks. An essential question that has always been ignored is how to design a fault tolerance scheme to buffer against cascading failures. In order to deeply understand the relationship between the fault-tolerance assignment and the robustness of interdependent networks, we propose a cascading failure model with the consideration of the fault tolerance property of the nodes in interdependent networks, where the node are specified different fault tolerant capacity according to their degree values. By theoretical and numerical studies, it is found that the robustness of a system can be improved significantly if the high-degree nodes are assigned high fault tolerant capacity. However, robustness of a system can be hampered if the high degree nodes are assigned low fault tolerant capacity. Our results prove that the fault tolerance assignment scheme of the nodes has great impacts on the robustness of interdependent networks and provide important thoughts to reinforce reliability of the interdependent systems by the proper allocation of fault tolerant capacity.

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.

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

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.

This paper proposes a hybrid ac/dc micro grid to reduce the processes of multiple dc-ac-dc or ac-dc-ac conversions in an individual ac or dc grid. The hybrid grid consists of both ac and dc networks connected together by multi-bidirectional converters. AC sources and loads are connected to the ac network whereas dc sources and loads are tied to the dc network. Energy storage systems can be connected to dc or ac links. The proposed hybrid grid can operate in a grid-tied or autonomous mode. The coordination control algorithms are proposed for smooth power transfer between ac and dc links and for stable system operation under various generation and load conditions. Uncertainty and intermittent characteristics of wind speed, solar irradiation level, ambient temperature, and load are also considered in system control and operation. A small hybrid grid has been modeled and simulated using the Simulink in the MATLAB. The simulation results show that the system can maintain stable operation under the proposed coordination control schemes when the grid is switched from one operating condition to another.

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.