HVDC Connection Research Articles

Distance Protection of Lines Emanating From Full-Scale Converter-Interfaced Renewable Energy Power Plants—Part I: Problem Statement

The fault ride-through (FRT) requirement of modern grid codes results in interactions between full-scale converter-interfaced renewable energy power plants (CIREPPs) and the protection systems of high-voltage transmission grids, which normally involve distance elements either as the primary or the backup relay. Such interactions are influenced by the CIREPPs' exclusive fault behavior, and have been left largely unnoticed in relaying literature. Part I of this paper develops a CIREPP model suitable for relaying studies, and highlights the CIREPP properties by which the protection system is endangered the most. Then, the operating scenarios leading to the malfunction of a distance relay that is located at a CIREPP substation and protects the adjacent line are unveiled. These scenarios include in-zone short circuits missed by the relay and incorrect tripping for out-of-zone faults, which would, in turn, neutralize FRT schemes implemented inside CIREPPs. The findings of this study also hold true for the ac lines emanating from voltage-sourced converter-based HVDC connections.

Optimal Design of the Electrical Layout of an Offshore Wind Farm Applying Decomposition Strategies

Electrical layout design is a key element in offshore wind farm planning, with a critical impact on both plant cost and reliability. Offshore Windfarm Layout optimizer (OWL) has been developed to efficiently find optimal electrical layouts in affordable computation times. The tool includes the possibility of HVDC connection and incorporates an approximation of losses, as well as stochasticity in wind inputs and component failures. OWL has been applied to several case studies including Barrow Offshore Wind Farm. OWL produces a significant cost reduction over the actually implemented design, with total realizable savings of EUR 800 k. The optimal layout includes redundant elements and deviates from a symmetrical pattern, therefore showing that a full optimization of the layout, rather than the selection of a pre-defined configuration, is necessary in order to fully capture efficiencies. The model relies on MIP and exploits the structure of the problem via decomposition strategies. Two different approaches have been developed, both resulting in substantial time savings. Benders' decomposition has been further improved by the addition of partially relaxed cuts and the application of scenario aggregation techniques. In addition, the Progressive Contingency Incorporation algorithm proposed by the authors is applied. Computation time savings reach two orders of magnitude.

Analysis of Grid Faults in Offshore Wind Farm with HVDC Connection

Future offshore wind farms are expected to be built farther away from shore and have larger capacities than today. This leads to new challenges related to grid connection. At distances longer than roughly 100km, HVDC transmission is preferred over AC transmission due to large charging currents in AC-cables. Conventional LCC (line commutated converter) HVDC is not suited for connection to weak grids like offshore wind farms, and the less mature VSC (voltage source converter) HVDC technology is preferred instead. A future large offshore wind farm with full power converter turbines and three-terminal VSC HVDC grid connection has been modelled in PSCAD. With three terminals the HVDC link can be used for direct transmission between the onshore terminals in addition to transmission of wind power. Due to the power electronics interfaces, the system has low short circuit capacity and is missing inertia. Also, DC-cables are discharged very fast during faults. This leads to different fault responses than in conventional grids. This work focuses on fault responses and protection of a HVDC-connected wind farm, both within the wind farm itself and in the HVDC-link.

Novel Integration of Wind Generator-Energy Storage Systems Within Microgrids

This paper proposes a novel integration of wind farm energy storage (ES) systems within microgrids. The main feature of the method is that the voltage and frequency control of the microgrid is shared by the wind generators through droop characteristics. Therefore, the control structure is suitable when the wind generators supply significant part of the load. The paper modifies the standard droop method in order to force the generators to share the load according to their available wind power and not just the generator rating. The paper focuses on islanded microgrid application; however, the method is applicable when there is a controlled connection to the main grid via a power converter, and also for HVDC connection. The proposed control methods are derived for doubly-fed induction generators (DFIGs) and validated using PSCAD/EMTDC simulations. The ride-through in case of a 3-phase fault on the local grid will also be simulated.

Control of an Off-Shore Synchronous Generator Based Wind Farm with Uncontrolled Rectifier HVDC Connection

Large off-shore wind farms based on synchronous generators can be connected to the mainland grid using modern wind turbines allow for tight control of terminal voltage. Therefore, an integrated design of wind turbine and HVDC link control can lead to a substantial simplification of the off-shore rectifier. This paper introduces an integrated control algorithm for line commutated thyristor (LCT) based HVDC transmission line whereby the off-shore rectifier tap changer is eliminated and a cheaper and more efficient uncontrolled rectifier is used instead of a LCT-based rectifier. The presented control strategy allows for off-shore ac-grid voltage and frequency control and shows good performance during steady state and transient operation, including on-shore fault ride-through operation.

PSCAD/EMTDC Modeling/Analysis of VSC-HVDC Transmission for Cross Border Power System Interconnection

When two adjacent AC systems operate at different frequency such as 50㎐ and 60㎐, as in the case of ROK and Russia, the only way to practically obtain the advantages of power system interconnection is to use HVDC connection. In this paper, application of the VSC-HVDC transmission for power system interconnection between Russia and ROK is evaluated through the PSCAD/EMTDC modeling and analysis. The simulation results show the feasibility of proposed system for cross border power system interconnection.

Large Offshore DFIG-Based Wind Farm With Line-Commutated HVDC Connection to the Main Grid: Engineering Studies

The paper considers a solution for integration of large offshore doubly fed induction generator-based wind farms with a common collection bus controlled by a STATCOM into the main onshore grid using line-commutated high-voltage dc connection. A design procedure is described and the controlled system is validated using PSCAD/EMTDC simulations confirming high performance of the proposed control strategy in both normal operation conditions and faults. Engineering issues related to STATCOM capacitor sizing and reduction of STATCOM rating are considered and their effectiveness is confirmed.

Control of Offshore DFIG-Based Wind Farm Grid With Line-Commutated HVDC Connection

The paper considers a control solution for integration of large offshore doubly fed induction generator based wind farms with a common collection bus, controlled by a static compensator, into the main onshore grid, using line-commutated high-voltage direct current connection. The paper's main focus is a mathematically grounded study of the power system interactions. That study produced an appropriate plant model for formal control design. A design procedure is described and the controlled system is validated using power systems computer-aided design/electromagnetic transient program simulations, which confirm the high performance of the proposed control strategy in both normal operation and fault conditions

HVDC Connection of Offshore Wind Farms to the Transmission System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents a technical and economic analysis to evaluate the benefits and drawbacks of grid connecting offshore wind farms through a dc link. A first case, concerning a 100-MW wind farm, is thoroughly investigated and cases of larger wind farms (200 and 500 MW) are presented. Three different transmission solutions are compared: 150-kV ac, 400-kV ac, and high-voltage dc based on voltage sourced converters (VSC-HVDC). After a brief overview of the features of these connection solutions, the related operational aspects are evaluated. An economic assessment compares the dc connection option to the ac alternatives, taking into account the investment, operation, and maintenance costs, and the negative valorization of losses and energy not supplied. Economic assessment includes sensitivity analyses of parameters, which could impact the 100-MW wind farm: distance, component costs, dc converter reliability, and dc converter losses. </para>

Control and Design of DC Grids for Offshore Wind Farms

Recently, the interest in offshore wind farms has been increased significantly. Besides the huge amount of space available, they have the advantage of an increased and more constant wind speed, leading to a higher and more constant production of power. Although, a lot of mechanical problems for constructing these farms have been solved during the past decade, the electrification is still a major issue in research. For large wind farms situated more than 60 km from the shore, an HVDC connection is favorable. Consequently, a pure dc system could be an interesting and cost-effective solution for offshore wind farms. In this paper, a control structure for such a dc grid is presented and analyzed. Furthermore, different configurations are compared and the developed simulation models are presented.

Technology of Offshore Wind Turbines and Farms and Novel Multilevel Converter-Based HVDC Systems for Their Grid Connections

The technology associated with offshore wind farms is discussed in detail. First, the various offshore wind turbines are reviewed and the factors influencing their characteristics are outlined in comparison with their onshore counterparts. This overview serves as a basis for the discussion that follows regarding the possible electrical connection within the farm, and between the farm and the grid. Voltage-source converter-based HV DC connection is compared with HVAC connection. Finally, a novel multilevel converter-based HVDC system, based on flying capacitor multilevel converters is proposed, as a possible interface between the farm and the grid.

Automatic generation control in a deregulated power system

Load frequency control (LFC) has been used for many years as part of the automatic generation control (AGC) in power systems around the world. In the synchronous Nordic power system, however, this function (termed secondary control) has so far been handled with manual control actions. Increased operational strain due to new HVDC connections in the next decade will make it increasingly difficult to maintain the current manual control system. In this paper a model of the interconnected power systems of Norway and Sweden is used to show how introduction of AGC might aid the system operator in handling the increased strain. However, the classical LFC based on the area control error is difficult to implement in a deregulated environment. An alternative concept is thus introduced where selected units are automatically following load changes on the HVDC connections. This ramp following controller (RFC) supported by manual control seems to be a promising option.

Technical and economic aspects of using a long submarine HVDC connection for frequency control

Several new submarine HVDC connections between the hydro-dominated Norwegian power system and the thermal-dominated UCPTE system of Western Europe are being planned, giving a total transfer capacity of approximately 3000 MW. Both existing and new HVDC exchange agreements are based mainly on the pumped storage principle, where Norwegian hydropower is exported during peak load, while surplus energy can be imported during off-peak periods. This paper focuses on an additional benefit of these HVDC connections: the possibility to supply frequency control in the thermal power system of northern UCPTE. Typical costs related to frequency control in thermal generating units are estimated, giving a strong economic motivation for this possibility. Furthermore, it is shown that the substitution of thermal frequency control reserves with a long submarine HVDC connection is possible without additional equipment or extensive control strategies, and without causing large disturbances in neither the thermal nor the hydro system.

Wide-area protection against voltage collapse

A wide-area protection system against long-term voltage collapse was designed, implemented, tested, modified, and commissioned. The system is based on the present SCADA system in southern Sweden, which has been complemented with input signals and equipment to execute action orders from the protection system. Bus voltages from the transmission system, reactive power output from generators connected to the transmission system, and current limiter information from main generators were used as input signals to the wide-area protection system. Actions from the protection system are shunt reactor disconnection, shunt capacitor connection, start of gas turbines, emergency power request from the HVDC connection to Germany, low priority load disconnection, and finally shedding of high priority load.