HVDC Terminals Research Articles

A very fast and easily implementable support vector machine based relay algorithm to classify the fault and non fault disturbance in VSC HVDC terminals

A very fast and easily implementable support vector machine based relay algorithm to classify the fault and non fault disturbance in VSC HVDC terminals

Operation of a Modular DC–DC Converter for Hybrid Interconnection of Monopolar and Bipolar HVDC Links

Multiterminal high-voltage dc (HVdc) grid has huge potential in long-distance bulk power transmission. Such HVdc structures have reached greater heights due to the concept of hybrid interconnections in HVdc by promoting power-sharing between different technologies of HVdc terminals. Modular multilevel converter (MMC) technology has further paved its way to act as a high-power HVdc transformer, which has encouraged the invention of many dc–dc MMC topologies. This article furthers the operation of an existing modular dc–dc converter for hybrid interconnection by proposing philosophies that establish its operation when one of the poles of an HVdc terminal is removed while the other terminal is bipolar. Moreover, hybrid arms, comprising of both full-bridge and half-bridge submodules are used in the converter, instead of only the full-bridge, to optimize the number of devices. The effectiveness of the proposed philosophies is demonstrated in multiterminal operation by simulation in the MATLAB/Simulink platform. The performance of these philosophies is also verified in a down-scaled laboratory prototype.

Enhanced grid frequency support by means of HVDC-based load control

The two major frequency incidents in Continental Europe and UK call for faster frequency regulation. Hvdc systems can dynamically adjust the power output to support the frequency regulation by means of droop characteristics. Adding derivative terms in the power controller enables inertia emulation that improves the system initial response. As drawback, due to the fast power transfer variation, the HVDC-connected areas suffer from temporary power imbalance and hence frequency deviation. To overcome this limitation, HVDC-based load controllers can be implemented at HVDC terminals to shape the consumption of nearby voltage-sensitive loads by means of controlled voltage variations, with the goal to minimize the frequency deviation in the supporting areas. In multi-terminal systems, the frequency support can be optimally shared among terminals depending on the load sensitivity. This minimizes the voltage deviation in supporting areas and limits the hvdc reactive power, while providing the same frequency damping in the faulty area.

Primary Frequency Regulation Using HVDC Terminals Controlling Voltage Dependent Loads

HVDC can provide frequency regulation during disturbances (e.g., faults) by controlling the power flow between two remote AC areas. While this action reduces the power deviation in the area affected by the disturbance, it causes a power imbalance in the other healthy AC area, leading to a frequency variation and endangering the system stability. In this work, a HVDC primary frequency regulation controlling voltage-dependent loads (PFR-VDL) is proposed, where the HVDC terminal in the healthy area influences the grid voltage amplitude to shape (decreasing or increasing) the load consumption in order to cope with the power variation required by the fault-affected area. The PFR-VDL extracts the needed energy for the frequency support, not from the generators (with following frequency deviation) but from the voltage-dependent loads in the healthy area. This work analyzes the PFR-VDL performance, generalizing it with two possible HVDC connection cases: Asynchronous connection with single HVDC line, and embedded HVDC forming a parallel, hybrid connection with HVAC. The PFR-VDL application benefits and limitations are evaluated analytically and verified by means of PSCAD EMTDC simulations, and finally validated with a large interconnected IEEE 39 bus system.

Coupling of AC Grids via VSC-HVDC Interconnections for Oscillation Damping Based on Differential and Common Power Control

This article presents a control approach for HVdc interconnections that provides damping of frequency oscillations in asynchronous ac grids by introducing a virtual friction for coupling their inertial dynamics. The HVdc interconnection is modeled by using the concept of common and differential power flow, allowing for independent control of the dc voltage and the net power transfer between HVdc terminals, respectively. The proposed controller introduces a damping effect in the differential power flow, which is equivalent to a mechanical friction between the generators connected to the ac grids at the two terminals. This virtual friction-based damping can effectively attenuate poorly damped frequency oscillations that can be observed, where the HVdc interconnection is interfaced to either of the ac grids without relying on fast communication between the converter terminals. The impact of the proposed control technique on the stability and damping of two interconnected power systems is first analyzed by using a simplified model. Then, the sensitivity to the frequency and damping of the oscillation modes appearing in the ac grid frequencies, as well as the effect of the dc line resistance on the oscillation damping are evaluated. Finally, the control system performance is experimentally validated on a scaled laboratory setup.

A New Filtering Scheme for HVDC Terminals Based on Damped High-Pass Filter

This paper presents the design and application of a recently developed damped high-pass (DHP) filter for the line-commuted converter HVDC terminals. The DHP filter can damp resonance at the noncharacteristic harmonic frequencies commonly encountered by multipulse HVDC links. A design method is developed to address two unique issues faced by HVDC filter design: a wide range of system impedances seen by a HVDC terminal and a variable reactive compensation configuration at the terminal. Both are challenging filter design problems even for traditional filter banks. Using realistic industry case and sensitivity studies, this paper has shown that the proposed filtering scheme and design method can result in saving in filter costs and space requirements without sacrificing performance in comparison with the common filtering schemes used at HVDC terminals. In addition, the proposed design concepts are also applicable to the design of other filters facing the same design issues.

Coordinated Control Strategies for Offshore Wind Farm Integration via VSC-HVDC for System Frequency Support

Coordinated control strategies to provide system inertia support for main grid from offshore wind farm that is integrated through HVdc transmission is the subject matter of this paper. The strategy that seeks to provide inertia support to the main grid through simultaneous utilization of HVdc capacitors energy, and wind turbines (WTs) inertia without installing the remote communication of two HVdc terminals is introduced in details. Consequently, a novel strategy is proposed to improve system inertia through sequentially exerting dc capacitors energy and then WTs inertia via a cascading control scheme. Both strategies can effectively provide inertia support while the second one minimizes the control impacts on harvesting wind energy with the aid of communication between onshore and offshore ac grids. Case studies of a wind farm connecting with a HVdc system considering sudden load variations have been successfully conducted to compare and demonstrate the effectiveness of the control strategies in DIgSILENT/PowerFactory.

Hybrid Simulation of Large Electrical Networks With Assymmetrical Fault Modelling

Owing to the increasing attention placed on dynamic security assessment in the light of recent blackouts, hybrid simulation, involving the interfacing of electromagnetic transients (EMT) simulators and transient stability (TS) simulators for the more accurate representation of very large electric networks, has been attracting considerable attention lately. The goal of hybrid simulation is to represent complex systems, such as FACTS devices and HVDC terminals, in detail, while ensuring an acceptable representation of the complete system such that control systems and switching devices are well represented. There has been significant success in this area. Most approaches, however, concentrate on contingencies that preserve the symmetry of the system. Asymmetrical faults, for example, have not been dealt with in the literature on hybrid simulation. In this paper we deal with how two very different simulators can be coordinated to work together and produce accurate solutions, even with asymmetrical disturbances. Based on the interaction protocol proposed in the paper, case studies were performed on 9-bus systems and 39-bus systems to assess the performance of the integration on both EMT and TS aspects. Very good results were achieved.

Sequence impedance and equivalent circuit of HVDC systems

This paper considers the sequence impedance and equivalent circuit of an HVDC converter and system. The results can be used in the steady state analysis of power systems containing HVDC terminals with systems operating under balanced or unbalanced conditions. The effects of commutation overlap are considered. The method described provides a simple model for steady state studies for power systems containing converters and HVDC links.

Effect of a modulated HVDC link on power system transients

This paper presents the details of a digital computer study of the effects of transient interactions, during large system disturbances, between an AC system and an associated high voltage DC transmission line with the rapid change in power transfer capability (modulated HVDC). The combined system consists of a large turboalternator synchronized to an infinite bus through two long-distance high voltage AC lines in parallel with an equally long controlled HVDC link. In the analysis, a rigorous nonlinear mathematical model, based on Park's equations, is formed by transforming the machine variables and the AC and DC transmission line parameters to a rotating reference frame attached to the alternator rotor, thereby providing the detailed transient behavior of the interconnected AC/DC system. Results of the studies demonstrate that a modulated HVDC link can provide significant increases in the loadability of AC lines by reducing or eliminating the constraints imposed by transient stability limitations. It appears, however, that, for certain systems, adverse stability behavior may result if the modulation setting exceeds 50%. Possible effects of HVDC modulation on the incidence of turboalternator torsional interactions are also considered. These interactions, however, are dependent upon parameters such as AC transmission, configuration, and the control mode characteristics of the HVDC terminals.

Transient AC voltage related phenomena for HVDC schemes connected to weak AC systems

A didactic explanation of voltage-stability-associated phenomena at HVDC terminals is presented. Conditions leading to AC voltage collapse problems are identified. A mechanism that excites control-induced voltage oscillations is shown. The voltage stability factor is used to obtain the maximum power limits of AC/DC systems operating with different control strategies. Correlation to Pd*Id curves is given. Solutions for eliminating the risks of voltage collapse and for avoiding control-induced oscillations are discussed. The results are supported by detailed digital simulations of a weak AC/DC system using EMTP. >

Subject index

A multiterminal HVDC system embedded in a conventional AC power system can be used as a control element for damping inter area oscillations and influencing the power flows in the AC network. The design of the multiterminal HVDC control system involves a large-scale, multivariable systems with the sensors and actuators geographically distributed. In this paper a new method for decentralized control system desion is proposed. The method allows the designer to quickly examine the decentralized control system and make modifications so the final control system satisfies the constraints and design objective. Besides its huge size and information constraints, one of the basic features of the multiterminal HVDC systems is that the parameters of the open-loop dynamics are not well known. In this light, robustness issue associated with multiterminal HVDC system are discussed. The theory is applied for decentralized controllers design for power system example which employ five HVDC terminals to damn out inter-area oscillations due to the AC power system dynamics.

A Computation Algorithm for Assessing Voltage Stability at AC/DC Interconnections

A new method for the analysis of ac voltage stability problems at HVDC terminals, when imbeded in large networks, is presented. The method can be used to evaluate the effects of dc modes of operation, ac system operating conditions and VAr compensation schemes on the combined system voltage stability. Dynamic properties of both ac and HVDC systems are incorporated in the solution algorithm. A step-by-step technique is developed for integrating the new algorithm with available load flow programs. Examples are given to demonstrate the performance of a new control strategy for HVDC converters connected to weak ac systems and the results are compared to classical solutions.

Advanced Scheme for AC Voltage Control at HVDC Converter Terminals

A new economic and effective scheme for controlling the ac voltage at the HVDC terminals during both steady state and dynamic conditions is presented. By extending the basic characteristics of an HVDC converter, it is possible to affect and modulate the reactive power balance on the ac side. Analytical studies as well as digital and simulator results are presented to demonstrate the viability of the proposed ac voltage control strategy to optimize HVDC terminal design and performance for a practical system. The economic and technical features of the new scheme are discussed and compared with other ac voltage control techniques such as synchrononous condensers and static VAr compensators.

Decentralized Control of Multiterminal HVDC System Embedded in AC Networks

A multiterminal HVDC system embedded in a conventional AC power system can be used as a control element for damping inter area oscillations and influencing the power flows in the AC network. The design of the multiterminal HVDC control system involves a large-scale, multivariable systems with the sensors and actuators geographically distributed. In this paper a new method for decentralized control system desion is proposed. The method allows the designer to quickly examine the decentralized control system and make modifications so the final control system satisfies the constraints and design objective. Besides its huge size and information constraints, one of the basic features of the multiterminal HVDC systems is that the parameters of the open-loop dynamics are not well known. In this light, robustness issue associated with multiterminal HVDC system are discussed. The theory is applied for decentralized controllers design for power system example which employ five HVDC terminals to damn out inter-area oscillations due to the AC power system dynamics.

Comparing Fundamental Frequency and Differential Equation Representation of AC/DC

HVDC terminals are modeled in load flow and stability programs using certain relationships for real and reactive power derived fran a set of assumptions for fundamental frequency voltages and currents.

Synthetic Test Installation for Converter Valves for HVDC Transmission

A synthetic valve test facility employing a unique low voltage off-commutation scheme has been constructed for the development and testing of Liquid Metal Plasma Valves for HVDC transmission. The installation is capable of testing valves up to a voltage and current of 200 kV and 1800 A, respectively. It possesses a high degree of operational flexibility and is able to closely simulate the working duty of converter valves used in HVDC terminals. The operating characteristics of the test installation have been illustrated.

Layout, Area, and Building Volume Requirements for HV DC Terminals

The different solutions for equipment layouts that are available in the construction of terminals for HV dc transmissions are introduced and discussed. Land area and the building volume requirements for the different layouts have been developed. These figures can be used as a base for system planning consideration and preliminary terminal cost estimates. Cost for premises and of building construction of the terminal buildings are also discussed. It is shown that the variation in this respect is considerable from location to location so that the most suitable layout will vary from case to case. Four alternative layouts are introduced. From the figures of these it is possible to select the type of layout which should be applied in a specific case.