Multi-terminal HVDC as enabling technology of premium quality power park

With the growing maturity of power electronic devices, the applications of the voltage source converter (VSC) technique on the high-voltage direct current (HVDC) transmissions have attracted great attention. Due to the inherent control characteristics of the VSC, the realization of multi-terminal HVDC (MTDC) grids becomes less complicated. However, one significant issue that restrains the development of VSC MTDC grids is the proper management against DC faults. This paper proposes a DC fault management scheme, delayed-auto-re-configuration (DARC), for VSC MTDC systems. This scheme does not require additional component to be added to the existing system. With appropriate protection and control, the MTDC system can be well protected and after the isolation of the faulted line section, the unfaulted terminals will be controlled and recovered to form a new MTDC configuration. The effectiveness of the proposed scheme is validated by simulations using the real-time digital simulator (RTDS).

Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control

The stabilization problem of multi-terminal high-voltage direct current (MT-HVDC) systems feeding constant power loads is addressed in this paper using an inverse optimal control (IOC). A hierarchical control structure using a convex optimization model in the secondary control stage and the IOC in the primary control stage is proposed to determine the set of references that allows the stabilization of the network under load variations. The main advantage of the IOC is that this control method ensures the closed-loop stability of the whole MT-HVDC system using a control Lyapunov function to determine the optimal control law. Numerical results in a reduced version of the CIGRE MT-HVDC system show the effectiveness of the IOC to stabilize the system under large disturbance scenarios, such as short-circuit events and topology changes. All the simulations are carried out in the MATLAB/Simulink environment.

In this paper, a lifetime estimation method for underground cable systems is proposed by combining a health index (HI) and conditional factor (CF). The underground cable system consists of underground cable, joint, termination, manhole, and duct bank. The HI is an indicator to indicate the actual condition of underground cable components and systems whereas the CF is used to indicate different operating stresses of the system under different operating conditions such as percentage loading, electrical stresses, laying structure, environment, etc. The actual technical data as well as historical operating and testing records are applied. The weighting and scoring method with the analytical hierarchy process are used to classify an importance of underground cable components, testing methods, and criteria used in the HI and CF calculation. The annual calculated HIs are plotted to investigate the lifetime trending curve by using a polynomial function. The degradation curve based on calculated CF is estimated by using the Weibull distribution function. Finally, the remaining life of the underground cable system is determined by matching the lifetime trending curve with the degradation curve. Ten practical underground cable systems supplying power in a high voltage power delivery system are evaluated with effective results. The lifetime of the underground cable system can be successfully estimated.

Converting existing ac transmission lines to an extended multiterminal three-wire bipolar HVDC system can be considered a cost-effective way to interconnect dispersed offshore wind farms to onshore ac grids instead of building new bipolar HVDC systems. To automatically coordinate between different converters in a multiterminal HVDC (MTDC) system, droop control techniques have been adopted as an effective means without the need for fast communications between units. The droop control design is mainly dependent on the line resistances. This paper shows that the equivalent resistance of a three-wire bipolar system changes based on the operational mode. The modification to droop control design of an MTDC equipped with a three-wire bipolar system is then presented to tackle this resistance variation with the operating condition. Two types of MTDC systems are considered in this work, namely, radial parallel and meshed parallel systems. Different simulation studies have been conducted to validate the results of the presented analysis.

A unified power-flow algorithm of AC power systems integrated with multi-terminal high-voltage direct current (MTDC) grids incorporating the interline DC power-flow controller (IDCPFC) is presented in this paper. The MTDC grids are based on voltage-sourced converter (VSC) technology. Pulse-width modulation control is employed for the VSCs. As against most of the research works published in this area, in this algorithm, the VSC modulation indices are considered as unknowns. The IDCPFC considered for the power-flow management of the DC grid is a generalized one, with an arbitrary number of DC voltage sources. In the proposed model converter losses are also included. All case studies are carried out in MTDC grids embedded in the IEEE-300 bus test system. For all the case studies, the power-flow algorithms were implemented with MATLAB. In all occurrences, a mismatch error tolerance of $$10^{-10}$$ p.u. was selected. Results obtained with case studies on AC–MTDC systems validate the proposed work.

Multi-terminal High-Voltage Direct Current (MTDC) networks are emerging as the promising technology in modern transmission systems for efficient and bulky renewable energy sources transfer over long distances. Nonetheless, the computational burden introduced for modeling MTDC networks with detailed models is heavy due to the inherent switching events of the Voltage Source Converters (VSCs). Transient system-level studies with less simulation time step-size can be achieved with Average-Value Models (AVMs). This paper delivers step-by-step modeling of the MTDC network with AVM -VSC in a systematic approach for steady-state and dynamic operation studies using Matlab/Simulink platform. The interaction between the MTDC network hierarchical control layers with the AVM -VSC model is covered and developed for a 4-terminal radial MTDC network. The DC voltage droop control is applied for the receiving end converters for the purpose of transmission losses optimization and to achieve power-sharing according to the converters ratings.

This paper proposes a control structure of a Multi-Terminal HVDC (MTDC) system employing Voltage Source Converters (VSC) technology. The study focuses on high-level control to keep DC voltage stable, mitigate oscillations due to the change in incoming power produced by contingencies in the AC grid and to provide the system with an adequate dynamic response [1]. Apart from this, controllers need to be validated in terms of HVAC grid compliance and for this purpose the UK National Grid Code will be used for verifying the fulfilment of frequency/voltage response and Low Voltage Ride-Through (LVRT). Thus, different tests will be performed considering the MTDC control objectives and the Grid Code compliance.

Summary form only given. This paper studies the techniques for modeling modular multilevel converter (MMC) based multi-terminal HVDC (MTDC) systems in the electromechanical transient mode. Firstly, the mathematical model of the MMC and its corresponding equivalent circuit are established, which are similar to those of the two level converters. Then, a power flow calculation method for AC/DC systems containing MMC-MTDC systems is developed. Two dynamic models for MMC-MTDC systems are developed in the paper. One is the detailed model, taking into account of the AC side circuit, the inner controllers, the modulation strategies, the outer controllers and the MTDC circuit. The other is the simplified model, which only reserves the outer controllers and partial dynamics of the MTDC circuit based on a quantitative analysis of the detailed model's dynamic processes, and it can be used in electromechanical transient simulation with a larger step size. Both the detailed and the simplified models are implemented on PSS/E and compared with the accurate electromagnetic transient models on PSCAD in a four terminal MMC-MTDC system; the result proves the validity of the developed models. Lastly, a stability study of a modified New England 39-bus system is executed, and the result shows that the AC fault can be isolated well in an MMC-MTDC asynchronously connected AC grid.

The 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.

High voltage DC (HVDC) transmission systems have demonstrated to be a cost effective solution for transmission of electrical energy over long distances or submarine and underground cables. Among the different HVDC technologies, voltage source modular multilevel converter (VSC-MMC) provides the highest power quality, modularity, availability and, additionally, it enables the implementation of multi-terminal HVDC (MTDC). These MTDC systems require a control system which can manage the power exchange among different HVDC stations keeping the DC voltage between certain limits. Previously proposed controllers, such droop control, usually requires adaptive algorithms to tune the controller parameters. On the other hand, constant dc voltage control provide low reliability because only one station controls the voltage of the complete dc line. In this paper, the control of an MTDC system based on VSC-MMC stations is proposed. A set of power and voltage limits are defined for each station, as well as the information related to allowed operation modes. The controller in one station does not require information from other stations because its operation mode and references depend on its own local variables. Simulation results which validate the proposed control strategy are shown.

DC line faults raise significant concerns in multiterminal high voltage direct current (MTDC) networks. The fault current ascends rapidly with a sharp peak, but isolating the entire faulty part is a non-sustainable solution for this matter. Therefore, rapid detection and classification of faults with high accuracy is indispensable. In this paper, a protection scheme is presented to protect MTDC networks based on voltage source converter (VSC) for interconnection of renewable energy sources, which incorporates main protection and post fault analysis. The main protection uses the Successive Data Window Technique (SDWT) as a fault detector, later on verifying it with the harmonics of the system voltage so as to prevent maloperation of the breakers, while the post fault analysis makes use of the voltage drop to classify the nature of fault, in addition to providing a backup in case the prior fails. The simulations results validate the protection scheme, which precisely identifies the fault and classifies its nature effectively. The performance of the proposed scheme is evaluated in PSCAD for MTDC networks. Matlab also verifies the results making this scheme a promising one.

Each year, millions of people and thousands of businesses are impacted by underground cable system failures. Underground cables are considered critical equipment within any power system, and typically one of the most expensive components of the system to repair. When they fail, the customer impact is immense and has the potential to cause severe collateral damage or worse, public safety concerns. Replacing underground power cables can be very expensive and time consuming and can take months or even years when associated with significant design, civil and construction work. Over 99% of solid dielectric (i.e.: XLPE or EPR) cable system failures are associated to Partial Discharge (PD). This paper characterizes the waveforms of Partial Discharge (PD) time domain signals utilizing a unique dataset of measured conditions of underground power cable systems. Machine Learning and Deep Learning models have been developed and evaluated for the purposes of providing the foundation for automated condition monitoring and predictive maintenance. The results demonstrate a step towards a predictive maintenance approach for underground cable systems.

Multiterminal high voltage direct current (HVDC) systems are developing rapidly lately. However, the high rising rate of fault current and the lack of interrupting capacity of the available DC circuit breakers hinder the further development of multiterminal HVDC systems. Being aimed at the challenge, this paper proposes a novel inductive DC superconducting fault current limiter (SFCL) to cooperate with available DC circuit breakers to cope with the short-circuit fault current in a multiterminal HVDC system effectively. In the normal power transmission of a multiterminal HVDC system, the SFCL presents low impedance and enables a smooth reclosing operation after an interruption. It is able to make a maximum use of the permeability of the iron core to offer a large inductance to curb the rising rate of the fault current in the early few milliseconds as a fault takes place. In addition, it can have an adjustable current limiting threshold to match the different requirements of different grid protection schemes. In this paper, first the working principle of the proposed SFCL is introduced. Then the experimental results on a concept-proofing prototype are reported and analyzed. The experimental results have verified the principle and functional ability of the proposed SFCL.

The purpose of this study is to improve the P-I controllers of the voltage-source converters (VSC)-based multiterminal high voltage direct-current (MT-HVDC) networks. Since the VSCs are the non-linear elements of the MT-HVDC stations, the classical optimization methods, which approximately implement the linear model to optimize the P-I controllers of the VSCs, do not generate optimal results. Therefore, this paper presents a novel technique to optimize the VSC-based MT-HVDC grids’ P-I controllers by embedding the artificial bee colony (ABC) algorithm. The voltage-droop control method is employed at on-shore grids to ensure the active and reactive power balance within MT-HVDC networks. During an evaluation, achieved via a detailed four-terminal MT-HVDC model designed in PSCAD/EMTDC, the improved results obtained under different dynamic situations such as unbalance wind power generation, change in load demand at the on-shore side grids, and eventual VSC disconnection, respectively.