Inverter AC System Research Articles

A novel evaluation method for the risk of simultaneous commutation failure in multi-infeed HVDC-systems that considers DC current rise

This paper proposes a novel evaluation method for the risk of simultaneous commutation-failures (CFs) in multi-infeed high-voltage direct-current (HVDC) systems, which takes into account the DC current rise caused by the inverter AC system fault. By calculating a multi-infeed interaction factor (MIIF), and using an equivalent model for multi-infeed HVDC systems. The effect of DC-current on commutation failure is analyzed and the factors that affect the simultaneous commutation-failure are determined. Based on the equivalent equation for both the DC line and DC control, a time-domain expression for the DC-current caused by the inverter AC system fault is calculated. Based on this, this paper describes a novel CF evaluation method that takes into account DC current rise by solving for the maximum of the DC current. Several useful parameters like the MIIF, the critical multi-infeed commutation failure factor (CMICFF), and the multi-infeed commutation failure factor (MICFF) are defined, and a method for the risk of simultaneous commutation-failure in multi-infeed HVDC systems by comparing CMICFF and MICFF is proposed. The accuracy of the method proposed in this paper is validated by a comparison with a simulation using the software PSCAD/EMTDC.

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.

Coordinated Control Strategy Based on MMC and DFIG

The AC-DC hybrid distribution network based on modular multilevel converters (MMC) is beneficial for receiving distributed generation (DG), energy storage devices and DC loads. Therefore, the coordinated control of MMC and DG is very important for its safe operation. In this paper, doubly-fed induction generator (DFIG) are used as DG, and the control strategies of DFIG and MMC systems are analyzed. The system modeling research is carried out on a real time digital simulator (RTDS) platform. Then based on MMC control, an improved adaptive Crowbar circuit is proposed to improve inverter AC system low-voltage ride-through (LVRT) capability. The results show that the coordinated control strategy can better compensate the reactive power of the system to achieve LVRT, and verify the effectiveness of the control strategy adopted by the converter station.

Simulation and analysis of cascading faults in hybrid AC/DC power grids

With the increasing use of DC transmission in existing AC networks, the possibility of AC and DC circuits running parallel to each other and sharing the same right-of-way or even the same tower is increasing. Cascading fault is a newly emerging fault type in hybrid AC/DC grids which severely threatens the power quality and safe operation of the network. Cascading fault is defined as an event which initiated from a fault in the inverter AC system or an abnormal operation of the DC system which leads to the multiple fault incidents in hybrid AC/DC power grids. In this paper, first, the mechanism and process of cascading fault in hybrid AC/DC grids are described. Subsequently, based on the analysis of several fault cases which may result in cascading faults in AC power grids, the typical characteristics of hybrid AC/DC power grids are demonstrated. The results from the simulation case studies in PSCAD/EMTDC indicate that the cascading fault is initiated from an inverter AC fault which results in a long-lasting communication failure, leading to a DC system block. Afterward, the power flow transfers from the blocked HVDC line to the paralleled AC lines which results in cascading trips in AC lines. Finally, some feasible solutions to prevent the cascading faults in hybrid AC/DC grids are pointed out.

An Integrated Control and Protection Scheme to Inhibit Blackouts Caused by Cascading Fault in Large-Scale Hybrid AC/DC Power Grids

Cascading fault is one of the serious challenges in hybrid ac/dc power grids, which initiates from a dc or a severe inverter ac fault and leads to a blackout in the inverter ac side. However, owing to the fact that dc faults do not cause commutation failure, the existing commutation failure inhibition approaches are not effective in the prevention of cascading faults caused by dc fault. In order to resolve the challenge, this paper first develops a hybrid ac/dc relay (HADR) based on the positive-sequence component, which can detect and locate the fault events in hybrid ac/dc networks. Subsequently, an integrated control and protection scheme is presented using the developed HADR and a thyristor-controlled series compensator. The proposed scheme has the ability to prevent the blackouts caused by cascading fault using transmission capacity enhancement of the ac line and load-shedding in the inverter ac system. The salient feature of the proposed scheme is that it provides a very economical way to compensate for the loss of power caused by HVdc line outage. In addition, it does not require communications among the relays. The practical performance and feasibility of the proposed scheme is validated by laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively inhibit the blackouts caused by cascading fault in hybrid ac/dc networks.

Impedance-Based Resonance Analysis in a VSC-HVDC System

Resonances can limit power transfer in a voltage-source converter-high voltage dc (VSC-HVDC) system. The objective of this paper is to develop impedance models for the rectifier ac system and the inverter ac system for a VSC-HVDC system. Resonance stability will be examined using Nyquist stability criterion and impedance frequency responses. The impedance models consider the outer control loop and the inner current control loops of VSCs. Impacting factors are then examined. Stability analysis demonstrates that the feedforward filter, line length, and power transfer level have a significant impact on resonances. Time-domain simulation results obtained by Matlab SimPowerSystems are used to validate the analysis.

A COMPARISON OF INVERTER CURRENT CONTROL AND RECTIFIER CURRENT CONTROL FOR HVDC SYSTEMS

This article presents a comparison between traditional rectifier current control and a proposed inverter current control for HVDC systems, considering both dynamic and functional steady-state aspects. The analytical system model, implemented in MATLAB, is used for the eigenvalue analysis and studies -based on the time domain performance index. It is shown that inverter current control has noticeable dynamic benefits, particularly for long cable DC systems and for weak inverter AC system configurations. Conventional rectifier current control is superior only with systems having very weak rectifier AC systems. Mode transition analysis fully confirms functionality of the inverter current control method but it also reveals particular new phenomena. A PSCAD simulation study with CIGRE benchmark model supplements salient results in this work.

Small signal analysis of HVDC-HVAC interactions

This paper presents a small signal analysis of interactions between HVAC and HVDC systems. The eigenvalue analysis, along with analysis of participation factors, is firstly described. The eigenvalue sensitivity analysis shows how the changes of AC system parameters influence the stability of the considered HVDC-HVAC system. The influence of short circuit ratio (SCR) changes on both the rectifier and the inverter AC system, is studied by examining the relative movement of the system eigenvalues. The most important conclusions about AC-DC interactions are obtained by investigating the nature of inherent feedback loops between the systems. By examining the changes of all interaction variables, it is determined which of the interaction variables should be controlled and which are better left uncontrolled.

Inverter controller for HVDC systems connected to weak AC systems

The paper presents a new control strategy for the inverter side of an HVDC system. The new control algorithm is intended for stability improvement of an HVDC system connected to a very weak receiving AC system. A root-locus analysis based on a detailed analytical system model determines the eigenvalues which are mostly affected by the reduction in strength of the inverter AC system. The feedback signal for the new controller is selected by considering possible movement of eigenvalues of importance. The controller is designed as a second-order filter placed in the feedback loop. Since the robustness of the system with respect to the AC system SCR changes is the main controller aim, the H∞ control theory is used for the controller design. Two cases have been studied; first, the design of a controller to improve the stability of an HVDC system connected to an AC system susceptible to large variations in its strength and secondly, the design of a controller which enables an HVDC system connection to a very weak AC system (SCR = 1.0 is considered). The PSCAD/EMTDC simulation results confirm that the controller meets the design objectives.

Unconventional HVDC Control Technique for Stabilization of a Weak Power System

A Constant Reactive Current Control (CRC) technique [1] was implemented in a fundamental frequency digital simulation. The interaction between the HVDC link and the associated AC system was analyzed and compared to the conventional current margin control method, for a very weak inverter AC system. The system studied has shown an unstable behaviour when the conventional method was used. By changing the control technique to the CRC method, system performance was significantly improved. It was also shown that with the CRC philosophy, a current modulation signal could be introduced without any harm to AC voltage stability. The CRC technique was also compared to a combined modulation-voltage and current-in a conventional control scheme, and showed a superior general behaviour.