The coupling of fundamental frequency voltages and currents on voltage-source converter (VSC)-HVDC overhead transmission lines from neighboring ac lines is analyzed. The complementary modulation components at dc and second harmonic frequencies on the ac side of the converter resulting from the fundamental frequency coupling are analyzed. A mathematical model is developed for the VSC converter that accurately represents modulation effects to and from its ac and dc sides. This model is shown to be significantly different from that of a conventional line-commutated converter. This model is validated by using electromagnetic transients simulation. The model is then combined with an accurate frequency-domain model of the transmission lines and the converter ac-side equipment, and used to conduct parametric analysis of the coupling effects. As the induced 0-Hz (dc) modulation component on the ac side is of serious concern, parametric plots are presented showing its variation with factors, such as ac/dc separation distance, paralleled length, transposition and VSC operating point.

Analysis of Coupling Effects on Overhead VSC-HVDC Transmission Lines From AC Lines With Shared Right of Way

The fault resistance is the key factor to affect the travelling waves and the performance of travelling-wave-based protections on HVDC transmission lines. In order to explain accurately the impacts of the fault resistance to travelling waves in theory and improve computational efficiency on the existing data of the travelling waves at given fault conditions, this paper proposed a global-sensitivity-based method to fast predict the new travelling waves with any other fault resistance. This method was built on the actual HVDC project topology and its frequency-dependent nature, and derived by the infinite-Taylor series in the phase and frequency domain, and then realized by a rational fitting technique with an order estimation scheme for the conversion to the time domain, which would overcome three limitations compared with conventional sensitivity methods. Numerical examples show that the proposed method has a huge advantage in saving CPU time to predict travelling waves in the network with any fault resistance increment over traditional simulations.

Global-Sensitivity-Based Theoretical Analysis and Fast Prediction of Traveling Waves With Respect to Fault Resistance on HVDC Transmission Lines

This paper presents a new HVDC system improved by an inductive filtering method, and afterward proposes a unified equivalent circuit model and a corresponding mathematical model for the calculation of the harmonic transfer characteristics of the new HVDC system. First, according to the special topology of the new HVDC converter station, an equivalent circuit model described by the fundamental and the harmonic impedances is established. Second, via the mathematical modeling of this new HVDC system, a harmonic transfer model is also established, which can be used to express the effect of inductive filtering method on the harmonic distribution characteristics. Moreover, by the theoretical analysis on the commutation process of the HVDC converter, a time-domain expression of the valve current is obtained to represent the harmonic characteristics of the converter. Finally, an experimental study validates the established analytical model and illustrates the harmonic transfer characteristics of the new HVDC system.

Harmonic Transfer Characteristics of a New HVDC System Based on an Inductive Filtering Method

A new DC current-voltage droop control method for a hybrid high-voltage direct-current (HVDC) system featuring a voltage-sourced converter (VSC) rectifier and a line-commutated converter (LCC) inverter is presented The proposed method utilizes DC voltage droop control to reduce the AC voltage fluctuation at the inverter side, to exploit the hybrid HVDC system for connecting offshore wind farms to weak grids without necessitating continuous reactive power compensation facilities Additionally, an optimal droop coefficient calculation method is proposed to keep the AC voltage constant for wind power fluctuations To evaluate the stability of the proposed method, a small-signal state-space (SS) model of the target system is derived and the root locus analysis is given Simulation case studies are performed using an electromagnetic transient model in PSCAD and an SS model in MATLAB The results demonstrate that a hybrid HVDC system with the proposed method successfully integrates offshore wind energy and weak grids without additional facilities because the AC voltage can be maintained at a near constant during wind fluctuations

DC Current and Voltage Droop Control Method of Hybrid HVDC Systems for an Offshore Wind Farm Connection to Enhance AC Voltage Stability

The line commutated converter (LCC) based HVDC projects generally use passive AC filters for harmonic cancellation, which has disadvantages of large footprint, poor filtering effect due to the offset of the resonance point, etc. This paper introduces a high-power LC hybrid active power filter (HAPF), which can meet the requirements of filtering and dynamic reactive power compensation. First, the passive part of the HAPF adopts two parallel single-tuned LC filters, which has low impedance at 12th and 24th. Second, considering the effects of the time delay of the high-voltage sensors, current controller and valve controllers, the possible negative impedance of the HAPF may bring a great risk of resonance and thus endanger the stability of the system. Third, an impedance model of the HAPF is established and the impact of the time delay on system stability is analyzed. Further, an adaptive resonance damping strategy based on least mean square (LMS) algorithm is proposed. Finally, the overall performances of the HAPF are verified by simulations on PSCAD/EMTDC.

The Use of the Hybrid Active Power Filter in LCC-HVDC Considering the Delay-Dependent Stability

This paper presents a set of equations for line commutated HVdc systems that can be used to calculate the main voltages and currents of positive or negative sequence on the ac side of the rectifier or inverter and to calculate the main voltages and currents on the dc side that are related to one ac system background voltage harmonic of positive or negative sequence. Furthermore, the frequency-dependent impedances of a line commutated HVdc system on the ac or dc side can be derived. It will illustrate how the converter impedances on both sides are related via an impedance transformation function, which is the product of the transfer functions for currents and voltages. The equations for the impedances differ from those given in a previous IEEE paper, especially with respect to the representation of the impedance for the converter transformers. It did, however, lead to similar results for the CIGRE benchmark model. In order to distinguish between a monopolar and bipolar scheme, the equations take the numbers of poles into account. Furthermore, six- or 12-pulse arrangements are considered.

Harmonic voltage and current transfer, and AC- and DC-side impedances of HVDC converters

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Harmonic voltage and current transfer, and AC- and DC-side impedances of HVDC converters