Multi-terminal Voltage Source Converter Research Articles

Dynamic Behaviour of Multi-Terminal VSC-Based HVDC after a Converter Outage: DC Control Strategy

aim of this paper is to evaluate the effect of DC-voltage control strategy on the dynamic behaviour of multi-terminal Voltage-Source Converter (VSC)-Based HVDC after a converter outage. In this paper, two dc voltage control strategies are considered: (i) standard voltage margin method (SVMM) and (ii) standard voltage-droop method (SVDM). The impact is evaluated in this paper using time-domain simulations on a simple test system using DIgSILENT ® PowerFactory TM considering a sudden disconnection of a converter-station. Simulation results demonstrate how important is the dc-voltage control strategy and the location/number of dc-buses involved in the dc-voltage on the dynamic response of the MTDC systems. The voltage margin control is capable of surviving a converter outage just if this converter is operating in constant power mode.

Integration of Large-Scale Offshore Wind Energy via VSC-HVDC in Day-Ahead Scheduling

This paper presents a security-constrained unit commitment (SCUC) solution for the optimal integration of large-scale offshore wind energy into a power grid. The solution considers a linear static state representation of multiterminal voltage source converter (VSC)-based high voltage direct current (HVDC) network and effectively incorporates this model into SCUC. The proposed algorithm consists of two major solution components: 1) component implements a SCUC solution for the inland power system; 2) component executes a scheduling of multi-terminal VSC-based offshore HVDC power system. Auxiliary problem principle technique is utilized to coordinate the power injections/withdrawals at HVAC/DC grid interfaces, which are obtained from both the inland and offshore scheduling components. Numerical tests illustrate the effectiveness of the proposed algorithm in the integration of large-scale offshore wind energy into the power grid.

Coordinated Operation and Control of VSC Based Multiterminal High Voltage DC Transmission Systems

This paper deals with a multiterminal voltage source converter (VSC)-based high voltage dc transmission system (M-HVDC) connecting offshore wind farms to onshore ac grids. Three M-HVDC configurations studied, each with different control strategies. The voltage-current characteristics of VSCs are presented and VSC converter operation with different output powers from offshore wind farms is assessed. A generalized droop control strategy is mainly used to realize autonomous coordination among converters without the need of communication. Operation of the three configurations with respect to their control system is analyzed through PSCAD/EMTDC simulations and experimentation. Results show control performance during wind power change, eventual permanent VSC disconnection, and change in power demand from the ac grid side converter.

Power Flow Algorithms for Multi-Terminal VSC-HVDC With Droop Control

This paper addresses the problem posed by complex, nonlinear controllers for power system load flows employing multi-terminal voltage source converter (VSC) HVDC systems. More realistic dc grid control strategies can thus be carefully considered in power flow analysis of ac/dc grids. Power flow methods for multi-terminal VSC-HVDC (MTDC) systems are analyzed for different types of dc voltage control techniques and the weaknesses of present methods are addressed. As distributed voltage control is likely to be adopted by practical dc grids, a new generalized algorithm is proposed to solve the power flow problems with various nonlinear voltage droops, and the method to incorporate this algorithm with ac power flow models is also developed. With five sets of voltage characteristics implemented, the proposed scheme is applied to a five-terminal test system and shows satisfactory performance. For a range of wind power variations and converter outages, post-contingency behaviors of the system under the five control scenarios are examined. The impact of these controls on the power flow solutions is assessed.

Optimizing DC Voltage Droop Settings for AC/DC System Interactions

In this paper, a methodology is presented to optimize the dc voltage droop settings in a multiterminal voltage-source converter high-voltage direct-current system with respect to the ac system stability. Implementing dc voltage droop control enables having multiple converters assisting the system in case of a converter outage. However, the abrupt power setpoint changes create additional stress in the ac system, especially when multiple converters are connected to the same interconnected ac system. This paper presents a methodology to determine optimized converter droop settings in order to not compromise the ac system stability, thereby taking into account the adverse effect the droop control actions have on the interconnected ac system. Developing a disturbance model of the interconnected ac/dc system, the principal directions indicate the gain and directionality of the disturbances; from this, optimal droop settings are derived to minimize the disturbance gain.

Voltage Control of Multiterminal VSC-HVDC Transmission Systems for Offshore Wind Power Plants: Design and Implementation in a Scaled Platform

This paper deals with multiterminal voltage source converter (VSC) HVDC transmission systems for the connection of offshore wind power plants to the main land ac grid. A droop-based control scheme is considered. The droop controllers have been designed base on a mixed sensitivity criterion by solving a convex optimization problem with linear matrix inequalities. The system is analyzed by means of simulations and experimentally in a scaled platform. Simulations show the control performance during a wind speed change and a voltage sag in the main ac grid. Experimental results include wind power changes (increase and decrease) and an eventual VSC loss (both considering grid-side and wind farm VSC loss). In all the cases, the simulation and the experimental results have shown a good system performance.

Benefits of Distribution-Level Power Electronics for Supporting Distributed Generation Growth

It is expected that distribution networks will be required to accommodate large amounts of distributed generation (DG). Keeping power flows and voltages within their limits will require either traditional infrastructure upgrades or active compensation. The form of active compensation (e.g., series, shunt, back to back, multiterminal), quantity, and rating of the compensator should be chosen to realize the best cost-benefit ratio. Distributed-generator and compensator placement algorithms are used with a power-flow and constraint satisfaction algorithm to analyze a large number of case studies (using real U.K. network data). From these cases, assessments of compensator performance are made and summarized statistically. When considering incremental deployment across all networks, with the site of greatest benefit chosen at each increment, it is found that static synchronous compensators provide the most favorable cost-benefit ratio. In contrast, multiterminal voltage-source converters tend to provide the greatest flexibility when considering uniform deployment across all networks. It is also observed that traditional reinforcement enhances the benefits provided by active compensation.

Generalized Steady-State VSC MTDC Model for Sequential AC/DC Power Flow Algorithms

In this paper, a steady-state multi-terminal voltage source converter high voltage direct current (VSC MTDC) model is introduced. The proposed approach is extended to include multiple AC and DC grids with arbitrary topologies. The DC grids can thereby interconnect arbitrary buses in one or more non-synchronized AC systems. The converter equations are derived in their most general format and correctly define all set-points with respect to the system bus instead of the converter or filter bus, which is often done to simplify calculations. The paper introduces a mathematical model to include the converter limits and discusses how the equations change when a transformerless operation is considered or when the converter filter is omitted. An AC/VSC MTDC power flow is implemented using MATPOWER to show the validity of the generalized power flow model.

Methodology for Droop Control Dynamic Analysis of Multiterminal VSC-HVDC Grids for Offshore Wind Farms

This paper addresses the control of multiterminal voltage-source converters at high-voltage direct current in the context of offshore wind farms. Droop control is commonly used to regulate the dc voltage in this kind of grid, and droop parameters are selected on the basis of steady-state analyses. Here, a control design methodology is proposed based on the frequency-response analysis. This methodology provides a criterion to select the droop gains, taking into account the performance specifications [i.e., the desired voltage errors and the maximum control inputs (currents)]. The application of the methodology is illustrated with a four-terminal grid.

Modeling and Simulation of Voltage Source Converter–Medium-voltage DC System for Stability Analysis

This article presents the modeling and simulation of a simple two-terminal and a multi-terminal voltage source converter based medium-voltage DC system, representing a simplified shipboard distribution network, for angular stability analysis. The control modes considered for the voltage source converter include the voltage control mode and the active and reactive power control schemes. The impact of these control modes on maintaining the DC voltage and, subsequently, on the transient as well as small-signal stability of the medium-voltage DC systems has been examined with the help of simulation in MATLAB/Simulink® (The MathWorks, Natick, Massachusetts, USA), and the results have been compared with an alternative medium-voltage AC distribution scheme. The impact of adding a power system stabilizer to enhance the small-signal stability has also been examined in the case of the medium-voltage AC architecture. Results indicate that that generators with the type 0 exciter are unstable with the medium-voltage AC architecture and stable with the medium-voltage DC architecture; using a type 1 exciter in the medium-voltage DC system improved the damping of the system. Use of a power system stabilizer for a medium-voltage AC system improved the damping, but the improvement was more significant with the placement of an static var compensator, in agreement with the results of participation analysis.