Multi-terminal HVDC System Research Articles

Sensitivity of Cable Model Parameters for Traveling Wave Differential Protections in MTDC Systems

Multi-terminal HVDC systems equipped with multiple DC breakers require a protection system that selectively detects faults within a few milliseconds. In the development and application of such protection algorithms, time-domain simulation studies of various faults are essential when determining the setting and verifying the performance. However, when such protections are applied in practice, it should be expected that the models will not perfectly represent the behavior of the real system or transmission line, thereby possibly causing protections to operate unreliably. This paper presents a simulation process that evaluates the consequences for a protection algorithm when the transmission line parameters are varied. More specifically, the false differential current due to cable model parameter errors is evaluated in a traveling-wave differential protection applied in a multi-terminal HVDC system during an external fault. The purpose is to identify the most critical parameters and evaluate how inaccuracies influence the protection performance when applied in practice. It is shown that the parameters which significantly influence the propagation delay are critical for achieving the ideal performance - a strictly zero differential current during external faults.

Non-Unit Ultra-High-Speed Line Protection for Multi-Terminal Hybrid LCC/MMC HVDC System and Its Application Research

The multi-terminal hybrid LCC/MMC HVDC has been applied to overcome the simultaneous commutation failure problem caused by the multi-infeed LCC-HVDCs in the East China. The high-speed and selectivity line protection method is required for the multi-terminal hybrid HVDC system. In this paper, the reflection and refraction of the fault traveling waves at different kinds of line terminals are characterized by transfer functions. Then the fault voltage as well as its first peak time at different line terminals when different internal and external faults occur are derived respectively. The conclusion proves that the first peak time of line-mode fault component voltage (FPTV) for internal faults is smaller than that for external faults, which is independent of fault impedance and fault type as well. Therefore, a FPTV-based non-unit ultra-high-speed dc line protection method is proposed. The algorithm is implemented in the developed protection prototype and validated in the Kun-Liu-Long three-terminal hybrid HVDC RTDS test system. The test results show that the proposed method can fast and correctly identify line faults in the multi-terminal hybrid HVDC system, even with 500 Ω fault impedance. Comparing with the traveling wave protection method of ABB, the proposed method improves the sensitivity when high-impedance faults occur.

Marginal Loss Modeling Based DCOPF and LMP Calculations for an Integrated AC and Multi-Terminal HVDC System

The objective of this work is to develop a framework for carrying out the locational marginal pricing (LMP) in a financially consistent manner for a generalized AC-DC system by considering the presence of multi-terminal high-voltage DC (MHVDC) links. Here, the financial consistency is sought in terms of having revenue adequate settlement within the multi-settlement framework in the presence of financial transmission rights. A suitable DC optimal power model is developed to perform the financially consistent locational marginal pricing for the generalized AC-DC system without compromising significantly with the power flow accuracy. Power losses in both the AC and DC networks are duly taken into account by using an upgraded version of the popular marginal loss modelling (MLM) approach. Apart from the incorporation of MHVDC links in the LMP based market clearing, provision is also created to receive bids and offers from entities that are directly connected to the DC network. Thus, separate sets of LMPs are generated for AC and DC networks in the AC-DC system. Ample case studies are performed with several differently sized systems to verify the effectiveness of the proposed MLM-based DCOPF framework for the LMP-based market clearing in a generalized AC-DC system.

Distributed Cooperative Voltage Control of Multiterminal High-Voltage DC Systems

Distributed Cooperative Voltage Control of Multiterminal High-Voltage DC Systems

Optimal power transmission in multi‐terminal HVDC systems for large offshore wind farms: a matheuristic approach

Nowadays power transmission losses in multi-terminal high-voltage DC (MT-HVDC) systems of large offshore wind farms are the main concerns to wind farm operators. Maintaining the DC voltages and currents within pre-specified limits and simultaneously optimising DC power flows of MT-HVDC systems using optimum droop control settings becomes a complex optimisation problem. This droop control scheme was less explored with power loss minimisation applications due to the difficulties in the optimisation of these droop gain settings. This study presents a new mathematical programming plus meta-heuristic-based optimisation approach to solve this optimisation problem. This hybrid optimisation methodology is also termed as 'matheuristic approach'. The proposed matheuristic method optimises the droop control settings for different operating conditions of offshore wind farms and simultaneously minimises the DC power losses in the MT-HVDC system. The proposed optimisation method has been tested upon a six-terminal HVDC system connected with large offshore wind farms. The superiority of the proposed optimisation method is demonstrated using the steady state as well as dynamic simulation studies. The simulation results show that DC power losses could be reduced significantly by adopting the proposed method to facilitate optimal power flows in the MT-HVDC system of large offshore wind farms.

Method for Determining the Droop Coefficients of Hybrid Multi-Terminal HVDC Systems to Suppress AC Voltage Fluctuations

Different from voltage-source converter (VSC), reactive power absorption of line-commutated converters (LCCs) changes depending on DC voltage level. Therefore, the amount of AC voltage fluctuations at inverter side are different according to $I$ – $V$ droop constants in hybrid multi-terminal high-voltage direct current (MT-HVDC) system. This paper proposes a method for calculating the $I$ – $V$ droop constants in a hybrid MT-HVDC system to suppress these AC voltage fluctuations, based on the sensitivity of AC and DC networks. Simulations performed using PSCAD demonstrate the effectiveness of the proposed method.

Primary Frequency Support Through North American Continental HVDC Interconnections With VSC-MTDC Systems

This paper proposes a frequency control strategy for voltage source converter based multi-terminal HVDC systems (VSC-MTDC) to facilitate the exchange of primary frequency reserves among asynchronous AC systems, thus providing frequency support from each AC system to the others. The proposed frequency control utilizes a reference signal calculated from global measurements which reflects the overall frequency dynamics of all connected asynchronous AC systems. The proposed control outperforms the traditional frequency droop scheme by reducing impact on the DC voltage profile and improving frequency nadir. The performance of the designed frequency control with different DC voltage droop controls is evaluated. The adaptation of the proposed frequency control for converter outages is analyzed. The robustness of the proposed control to communication latency and the sensitivity of frequency support to power limits are also investigated. The VSC-MTDC model and the proposed control are implemented in the commercial grade software PSS/E and thus is suitable to study large-scale realistic systems and can be incorporated into the power system planning process at utilities and ISOs. The effectiveness of the proposed control is illustrated on a developed AC-MTDC test system and also on a large-scale realistic model combining the North American Western Interconnection (WI) and Eastern Interconnection (EI) with continental HVDC interconnections.

A Novel DC Transmission System Fault Location Technique for Offshore Renewable Energy Harvesting

Advances in renewable energy recourses technologies have paved the way for increased utilization of DC grids in modern power systems. In particular, high voltage DC (HVDC) transmission cables are used to harvest offshore energy. Since failure diagnosis and repair of the buried offshore cables under the sea bed involve quite laborious tasks, accurate fault location is of vital importance. This paper proposes a novel fault location method based on the synchronized measurement of conductors and sheaths currents in each terminal without requiring the voltage signal. Unlike the traveling-wave-based methods, the proposed method is developed for multi-terminal HVDC systems without requiring any high precision measurement infrastructure. The proposed method can be implemented for different system topologies as well as fault resistance and fault distance. The fault location method can provide accurate results even if 10 ms time window samples are imported into the proposed algorithm. In addition, accuracy of the fault location using the proposed method is not affected significantly by noisy condition. Different test systems have been simulated to evaluate performance of the proposed method in various scenarios. Obtained results confirm that the proposed practical method can provide the accurate fault location for all different scenarios and conditions.

A Selective Fault Clearing Scheme for a Hybrid VSC-LCC Multi-Terminal HVdc System

A selective fault clearing scheme is proposed for a hybrid voltage source converter (VSC)-line commutated converter (LCC) multi-terminal high voltage direct current (HVdc) transmission structure in which two small capacity VSC stations tap into the main transmission line of a high capacity LCC-HVdc link. The use of dc circuit breakers (dc CBs) on the branches connecting to VSCs at the tapping points is explored to minimize the impact of tapping on the reliability of the main LCC link. This arrangement allows clearing of temporary faults on the main LCC line as usual by force retardation of the LCC rectifier. The faults on the branches connecting to VSC stations can be cleared by blocking insulated gate bipolar transistors (IGBTs) and opening ac circuit breakers (ac CB), without affecting the main line’s performance. A local voltage and current measurement based fault discrimination scheme is developed to identify the faulted sections and pole(s), and trigger appropriate fault recovery functions. This fault discrimination scheme is capable of detecting and discriminating short circuits and high resistances faults in any branch well before 2 ms. For the test grid considered, 6 kA, 2 ms dc CBs can easily facilitate the intended fault clearing functions and maintain the power transfer through healthy pole during single-pole faults.

Improved Coordinated Control of AVM based Multi-Terminal HVdc System using Steady-State Analysis

Improved Coordinated Control of AVM based Multi-Terminal HVdc System using Steady-State Analysis

Cost-Based Adaptive Droop Control Strategy for VSC-MTDC system

This study proposes a cost-based adaptive (CBA) droop control strategy for use in a voltage source converter (VSC)-based multi-terminal high voltage direct current (MTDC) system. Rather than using a fixed droop gain, we suggest the CBA droop control scheme, which reduces the total incremental generation cost of ac systems, while sharing the burden based on the available capacity of VSCs at the post-contingency-steady-state operating point. Following a certain VSC outage in the MTDC system, unbalanced power is allocated based on the equal incremental cost principle to reduce the total active power generation cost. The results were verified through a transient simulation on an MTDC system with four monopole VSCs, and outage contingency scenarios were presented with three groups of generation cost curves. The results indicate that the CBA droop control strategy can provide greater contributions to the economic operation of the MTDC system, while achieving robust control.

TLBO Algorithm for Multi-Level Inverter-Based Multi-Terminal HVDC System in Grid-Tied Photovoltaic Power Plant

The main objective of this paper is to overcome the problems with conventionally tuned proportional-plus-integral (PI) controller by using TLBO-PI technique, which enhances the dynamic performance. The significant problem is that it is not able to provide optimal parameters, which is overcome by new teaching-learning-based optimization (TLBO) algorithm. This research work develops a new TLBO for multi-level inverter (MLI)-based multi-terminal HVDC (MLI-MT-HVDC) in grid-tied photovoltaic power plants (GTPVPPs). In this, 7-level inverter is replaced by a conventional 3-level, due to its advantage of less number of switching components, less dc source requirement, low total harmonic distortion (THD) and due to main effective reason of application in HVDC transmission. Here two terminals with individual grid integration are interconnected to a common terminal of solar distributed generation (DG). Among those terminals with grid integration, one is faulty and other is healthy terminal, in which the healthy terminal is not affected due to the fault on a faulty terminal. A teacher and students in a class are employed in step-by-step manner to generate an optimal result. The test system is also controlled with a fuzzy logic controller (FLC), adaptive neuro-fuzzy inference system (ANFIS), but due to its simple structure and its application in industries, a PI controller with new optimization method is implemented. Additionally, it is clear that TLBO-PI can reduce the THD to the level as per IEEE 519-2014 standards, improve fault-ride-through (FRT) capability and enhance the dynamic performance better during abnormalities and dynamic load.

Coordinated Design of Supplementary Controllers in VSC-HVDC Multi-Terminal Systems to Damp Electromechanical Oscillations

High Voltage Direct Current Transmission (HVDC) systems are called to play an important role in modern electric power systems. Among the many aspects of HVDC currently under active research, this paper presents a proposal to design supplementary controllers in multi-terminal HVDC systems based on Voltage Source Converters (VSC-MTDC, for short) to damp electromechanical oscillations (Power Oscillation Damping or POD). In the proposed POD controllers (PODC), each VSC station compares the average of the frequencies measured at the AC connection points of the VSC stations of the MTDC system (frequency set point) with its own AC output frequency. This difference is then used to manipulate the active- (P) and reactive-power (Q) injections of each one of the VSC stations. The proposed PODCs are designed systematically by using a coordinated-design method using the concept of eigenvalue sensitivity, in order to achieve the required damping ratio of a set of electromechanical modes of interest in the power system where the VSC-MTDC is connected. The proposed designed methodology has been tested in the Cigré Nordic32A system with an embedded VSC-MTDC system. Results show that the proposed PODCs are effective damping successfully the electromechanical modes of interest and they are robust against communication latencies.

System operational dispatching and scheduling strategy for hybrid cascaded multi-terminal HVDC

A hybrid cascaded Multi-terminal HVDC (MTDC) system involving both series and parallel-connected converters, which combines the advantages of line commutated converter based (LCC) and voltage source converter based (VSC) HVDC for ultra-high voltage, large-capacity and flexible power transmission, is proposed and is to be implemented in a real project in China. Due to complexity of operational control and difficulty of operational scheduling brought by the hybrid cascaded MTDC system and the lack of prior related research works, this paper establishes the mathematical model of the hybrid cascaded MTDC system and then proposes a system operational dispatching and scheduling strategy, which includes: 1) feasible coordinated operational control strategies investigation; 2) security operating range of the whole hybrid cascaded MTDC system analysis; 3) operating points of the hybrid cascaded MTDC system calculation. Simulations results based on PSCAD/EMTDC are used to validate the proposed system operational dispatching and scheduling strategy.

Adaptive Droop Control of Multi-Terminal HVDC Network for Frequency Regulation and Power Sharing

This paper proposes a novel adaptive droop control strategy for multi-terminal HVDC (MTHVDC) network to provide frequency support and power sharing via a distributed consensus algorithm. The droop coefficient is adaptively tuned by an effective integration of optimization algorithms and frequency consensus method. The optimization control aims to minimize generation cost, frequency deviation cost, and converter losses. Furthermore, the coordination between the consensus-based droop controller and the optimization technique is established by a feedback loop of coupling gain solution method, which is based on coupling gain matrix. The proposed control is successfully applied to a four-terminal MTHVDC system and modified New England 39-bus system on PSCAD. Power sharing is further improved by adding an adjustment factor based on frequency deviation with coupling gain matrix, which redirects power flow in ac and dc sub-grids. In addition, the loss reduction of voltage source converters is presented to demonstrate the proposed control performance.

Distance protection algorithm for multiterminal HVDC systems using the Hilbert–Huang transform

Multiterminal high-voltage direct current (HVDC) systems still need advances in terms of protection in order to improve their reliability. In this context, the distance protection can play a major role by adding selectivity to the existing DC fault detection algorithms. Hence, the present work proposes a non-unit DC distance protection algorithm that uses the frequency of the DC voltage transient oscillation to estimate the distance of the fault. The DC voltage transient frequency is extracted using the Hilbert–Huang transform and compared with a pre-defined frequency/distance curve. The technique was evaluated by simulating faults in a four-terminal symmetric monopole multiterminal HVDC system. In the simulation environment the algorithm was fully selective for faults within the first protection zone and had a correct operation rate of 94% or more for faults located in the second protection zone. To further validate the presented technique, the proposed algorithm was embedded in a digital signal controller, running in real-time. In all performed tests in hardware, the faults were correctly detected and identified as being internal or external. The results indicate that the proposed algorithm could be used in real-world applications, in conjunction with fault detection techniques, adding selectivity to multiterminal DC protection schemes.

Analytic Quantification of Interactions in MTDC Systems Based on Self-/En-Stabilizing Coefficients in DC Voltage Control Timescale

Multiterminal high-voltage direct-current (MTDC) systems based on voltage-source converters (VSCs) are used for the transmission of large-scale renewable energy generations. Dynamic interactions among different VSCs through the coupling of DC and AC networks become one salient problem in an MTDC system. Self- and en-stabilizing coefficients, which are expressed analytically in the frequency domain with clear math and physical meanings, are defined to quantitatively describe and analyze the interactions of an MTDC system in this article. Some insights behind such interactions are concomitantly revealed by the analytic quantification. In this course, it not only points to the devices and interaction paths causing the oscillations but also illustrates how controller parameters affect the system stability by analytic expressions. This method provides an analytical way to quantify the interactions among VSCs in DC voltage control timescale, henceforth leaving a deeper understanding of the dynamics in an MTDC system.

SCIG Based Wind Energy Integrated Multiterminal MMC-HVDC Transmission Network

Modular multilevel converter (MMC) based HVDC system for renewable energy integration has attracted the researcher’s interest nowadays. This paper proposes a control strategy for MMC based multiterminal HVDC system for grid integration of squirrel cage induction generator (SCIG) based wind energy systems. Unlike the average model, this work models the MMC using the aggregate model and develops multiterminal HVDC transmission network in MATLAB/Simulink. It further develops the MMC multiterminal HVDC transmission network in real time digital simulator (RTDS). Instead of simplified current source, the proposed network considers the complete dynamics of SCIG based wind source from generation to integration. It employs field-oriented control for optimum wind energy tracking and forms isolated AC grids using feed forward controller. The proposed MMC controller performance has been tested under severe balanced and unbalanced disturbances. The results from aggregate model based MMC network in MATLAB/Simulink and those of the experimental MMC network in RTDS are in full agreement. The results confirm optimum wind energy tracking, fulfill grid code requirements, and improve low voltage ride through capability.

Distributed Voltage Regulation and Automatic Power Sharing in Multi-Terminal HVDC Grids

Conventional droop control methods for power-sharing in a multi-terminal high voltage DC (MTDC) grid lead to voltage deviation from the nominal value. Moreover, the power-sharing is inaccurate in the droop-controlled MTDC system. This paper proposes a secondary controller with a distributed architecture to compensate the voltage deviation and to achieve equal power sharing automatically among the converter stations. A distributed observer is used to estimate the average voltage of the converter stations. The proposed distributed controller and observer use local measurements of output powers and voltages, and only communicate with neighboring stations. Therefore, the requirement of the global information in the centralized secondary control schemes is eliminated, which reduces the communication requirement and improves the reliability of the MTDC grid. The case studies illustrate that the proposed distributed controller regulates the stations' average voltage and shares the power mismatch accurately. Impact of the communication time-delays and the control gain on stability of the MTDC grid are also investigated using the Lyapunov-KrasovskiiLMI condition. It is shown that reducing the control gain can stabilize the MTDC grid in the case of large communication time-delays.

VSC Controllers for Multiterminal HVDC Transmission System: A Comparative Study

Voltage source converter (VSC)-based multiterminal high-voltage direct current (HVDC) systems received widespread attention throughout the world for grid integration of renewable energy resources in recent years. This paper presents a comparative performance analysis of different VSC-based outer control and inner current controllers for the multiterminal HVDC system. It employs either lead-lag (LL) or proportional–integral (PI) controllers for outer DC link voltage control, whereas it uses PI or model predictive controllers (MPCs) for inner current control. Hence, it designs four combinations of controllers (LL–MPC, LL–PI, PI–MPC, and PI–PI) to control the outer DC link voltage and inner current of the VSC-based multiterminal HVDC system. Besides, it proposes an integral time squared-error (ITSE)-based optimization technique to tune the parameters of the employed PI controllers that selects optimal parameters at minimum ITSE under extreme operating condition. The combination of the mentioned controllers forms the main control unit of the multiterminal HVDC transmission network for regulation of the DC link voltage and the power flow. Moreover, this article evaluates the controller performance in terms of maximum overshoot, steady-state error, settling time, rise time, and total harmonic distortion. It implements the proposed controllers in a typical VSC-HVDC system and multiterminal HVDC transmission system in MATLAB/SIMULINK platform. Presented results confirm the efficacy of the four-type controllers. The optimized PI–MPC controller provides overall better performance relative to other controllers.