Multi-terminal DC Transmission Research Articles

Coordinated direct current matching control strategy for multi-terminal DC transmission systems with integrated wind farms

A new direct current matching control (DCMC) scheme is proposed in this paper. The scheme is ideally suited for the integration of a large number of wind farms with AC grid systems via a multi-terminal HVDC (MTDC) network incorporating several grid-side converters. The proposed DCMC, which matches, in a near-instantaneous fashion, the cumulative injected DC currents from all wind farms with the total of the output DC currents to the AC grids (via inverters) by communicating real-time data between all terminals, is an improvement upon and potential replacement for conventional DC voltage droop and master–slave control strategies. Through the utilization of a wide-area supervisory control and data acquisition (WA-SCADA) system, the proposed DCMC aims to enhance MTDC network voltage stability and facilitate flexible power dispatch to the supplied AC grids, while maximizing the total amount of generated wind power and offering more flexibility in terms of the ability for wind farms to independently control and maximize their outputs without any requirement for output to be constrained. A six-terminal MTDC system connecting three wind farms to three independent mainland AC grids is used to validate the proposed DCMC and compare its performance with conventional control strategies, three simulation studies are carried out to test and verify the DCMC.

A Hybrid MMC Topology with dc Fault Ride-Through Capability for MTDC Transmission System

This paper proposes a hybrid modular multilevel converter (MMC) topology based on mismatched-cascade mechanism. The blocking conditions of different submodule (SM) structures under dc fault are analyzed and a series double submodule is presented. With series-double submodules and mismatched-cascade submodules, the proposed hybrid MMC can ride-through the dc side short-circuit fault and provide an output voltage with the feature of low harmonic content. This hybrid MMC topology can be used in the VSC based multiterminal dc (VSC-MTDC) transmission system. The dc fault ride-through properties of the new structure and the total harmonic distortion (THD) are analyzed compared with the previous full-bridge and clamp-double architectures. An appropriate fault blocking procedure is presented, and a typical four-terminal dc transmission simulation system is given in the power system simulation software. Finally, simulation of steady-state and dc bipolar short-circuit fault verifies that the MTDC system based on this new hybrid MMC topology is stabilized and can block the dc fault and return the nonfault parts to normal.

Paralleled multi‐terminal DC transmission line fault locating method based on travelling wave

This study presents a novel protection criterion of fault locating in multi-terminal DC (MTDC) transmission systems. When a fault occurs on the DC transmission line of the MTDC system, the traditional protection cannot locate the fault because there is no smoothing reactor (equipped at the converter output) in the transmission lines. In the circumstance of AC/DC conductor contact (ADCC) fault, the varieties of the transient voltage and current components are given based on the symmetrical fault analysis. According to the Bergeron equation, the travelling wave characteristic is analysed. A new criterion of fault locating using the change rate of the current travelling wave is presented which can reduce the impacts of the related fault on the AC system. Finally, simulation results verify the effectiveness of the proposed protection criterion.

Fault location on multi-terminal DC systems using synchronized current measurements

Multi-terminal dc transmission systems are suitable substitutes for their ac counterparts, once the associated costs are justified. This paper proposes a new traveling wave-based fault location method for multi-terminal dc (MTDC) systems. In doing so, the fault section and its included lines are specified using a straightforward algorithm. Then, an evaluation index is developed to determine the faulted line amongst all included ones in the fault section. Finally, a system of linear equations taking into account the fault inception time, the fault location and the first arrival times at different detector locations are constructed and solved. The method merely needs the first current surge arrival times, thus eliminating the practical difficulties in relation with identifying other traveling wave features. Besides, the fault is easily located despite the complexity of the network topology, i.e., the number of its meshes and radial lines. To examine the effectiveness of the proposed method, a large number of simulation studies are conducted on a complicated MTDC system. Obtained results verify the high accuracy, noise immunity and fault impedance robustness of the proposed method.

A fault location criterion for MTDC transmission lines using transient current characteristics

Multi-terminal DC (MTDC) transmission system is applied in modern power systems due to its technical features. In an MTDC system, the converter stations are generally connected in parallel. To smooth the DC current output from the converter, the smoothing reactors are integrated in the converter outputs, instead of in the DC transmission line. When a fault occurs on the DC transmission line of the MTDC system, the fault current will flow along the whole DC line. As a result, the protection scheme may not locate the fault accurately. Based on the characteristics of DC filter, this paper analyzes the impedance characteristics of the combinational circuits, and proposes a method to identify the fault location by the characteristic harmonic. A protection criterion of the non-integer harmonics comparison is composed with phase-frequency characteristics curve derived from complex wavelet transform. The inevitable blind zone is analyzed in theory which exists on the terminal of transmission lines just like distance protection in AC systems. The algorithm is successfully tested and compared with other techniques.

An Adaptive Coordinated Control for an Offshore Wind Farm Connected VSC Based Multi-Terminal DC Transmission System

AbstractThe voltage source converter (VSC) based multiterminal high voltage direct current (MTDC) transmission system is an interesting technical option to integrate offshore wind farms with the onshore grid due to its unique performance characteristics and reduced power loss via extruded DC cables. In order to enhance the reliability and stability of the MTDC system, an adaptive neuro fuzzy inference system (ANFIS) based coordinated control design has been addressed in this paper. A four terminal VSC-MTDC system which consists of an offshore wind farm and oil platform is implemented in MATLAB/ SimPowerSystems software. The proposed model is tested under different fault scenarios along with the converter outage and simulation results show that the novel coordinated control design has great dynamic stabilities and also the VSC-MTDC system can supply AC voltage of good quality to offshore loads during the disturbances.

Small-Signal Stability Analysis of Multi-Terminal VSC-Based DC Transmission Systems

A model suitable for small-signal stability analysis and control design of multi-terminal dc networks is presented. A generic test network that combines conventional synchronous and offshore wind generation connected to shore via a dc network is used to illustrate the design of enhanced voltage source converter (VSC) controllers. The impact of VSC control parameters on network stability is discussed and the overall network dynamic performance assessed in the event of small and large perturbations. Time-domain simulations conducted in Matlab/Simulink are used to validate the operational limits of the VSC controllers obtained from the small-signal stability analysis.

Call for Papers: HVDC Transmission and Large Scale Energy Storage

European Transactions on Electrical Power will publish a Special Issue entitled ‘HVDC Transmission and Large Scale Energy Storage’. The Guest Editor for this issue will be Prof. Brian K. Johnson of the University of Idaho. Voltage source converter HVDC technologies and applications UHVDC (+/−800 kV) transmission technologies and applications Multiterminal HVDC systems DC transmission and distribution in urban power systems AC to DC line conversion Developments in power converter technologies for HVDC and energy storage Integrated storage and renewable generation in regional bulk power systems Applications of energy storage to improve AC/DC transmission system performance Use of HVDC systems and energy storage as shock absorbers to limit the propagation of major power grid disturbances The papers should be in English and of a maximum of 25 pages, including references and illustrations. All submitted papers will be reviewed by at least three reviewers and selected based on their originality, significance, relevance, and clarity of presentation. Completed papers should conform to the journal's style and according to the instructions for authors which can be found on the journal's homepage, under “For Authors”. http://www.interscience.wiley.com/journal/etep/home/forauthors.html. Papers have to be submitted online via the journal's homepage at the URL above, under “Online Submission”. Authors must initially select Prof. Deqiang Gan as the Submitting Editor. Each submission must be clearly identified as a submission for this special issue and the special issue code, HVDC, must be entered in the Special Issue Title field, when requested. Submissions due: 31 August 2008

Modeling of the Hydro-Quebec-New England HVDC system and digital controls with EMTP

The commissioning of the Quebec-New England multiterminal DC transmission system underscores the increasing need to develop numerical models for simulation purposes. The modern microprocessor-based HVDC (high-voltage direct-current) converter controls used in this scheme make the modeling a real challenge since the Electromagnetic Transient Program (EMTP), usually employed for simulation purposes, and Transient Analysis of Control Systems (TACS) were designed to simulate analog, not digital controls. Details on the modeling of digital controls with a modified EMTP version are given. Some comparisons with HVDC (high-voltage direct current) simulator results which show good agreement between the two power system analysis tools are presented. A case that is validated with field test results is also included. >

Consideration for Implementing Multiterminal DC Systems

The established principles of multiterminal dc transmission are applied to the planning needs of practical projects. Without further detailed review of operating techniques, possibilities and contraints that are intended to be useful in the assessment of potential projects are discussed. Reference is made to the plans for certain schemes and the status of dc circuit breakers is reviewed.

A new control method for multiterminal DC transmission

A new control scheme for parallel multiterminal dc systems is proposed. The scheme is divided into two separate functions: (1) a supervisory control called the System Monitor-State Predictor (SMSP), which determines the steady state operating point of the system from ac voltage measurements, power requirements at the converters, and the dc network configuration; and (2) the feedback controllers which drive the system to the desired operating point supplied by the SMSP. A systematic method is presented for calculating local feedback gains based on optimal control theory, and it is shown that use of feedback measurements is possible with negligible loss of dynamic performance. A detailed hybrid computer simulation of a four-terminal dc system was used to demonstrate the excellent performance of the new control scheme during steady state operation and for large system disturbances.

Development of centralized controller for multiterminal DC transmission systems

To operate a multiterminal dc transmission system stably and reliably, it is necessary to develop a centralized controller which transmits coordinated control signals to converter stations. A centralized controller for multiterminal dc transmission systems is proposed which monitors the operating condition of dc transmisson system, makes necessary logical judgement and issues necessary command signals. A control computer is able to fulfill the functions of the central controller if a suitable software system is provided which processes linkage between tasks, processing priority and task interruption. The proposed centralized controller of multiterminal dc transmission system operates very satisfactorily in all operating conditions including start, stop, power reversal and steady-state operation. (LCL)

直流多端子送電集中制御装置の開発

直流多端子送電集中制御装置の開発

直流多端子系統における短絡電流のしゃ断方式

直流多端子系統における短絡電流のしゃ断方式