HVDC Lines Research Articles

An Integrated Control and Protection Scheme to Inhibit Blackouts Caused by Cascading Fault in Large-Scale Hybrid AC/DC Power Grids

Cascading fault is one of the serious challenges in hybrid ac/dc power grids, which initiates from a dc or a severe inverter ac fault and leads to a blackout in the inverter ac side. However, owing to the fact that dc faults do not cause commutation failure, the existing commutation failure inhibition approaches are not effective in the prevention of cascading faults caused by dc fault. In order to resolve the challenge, this paper first develops a hybrid ac/dc relay (HADR) based on the positive-sequence component, which can detect and locate the fault events in hybrid ac/dc networks. Subsequently, an integrated control and protection scheme is presented using the developed HADR and a thyristor-controlled series compensator. The proposed scheme has the ability to prevent the blackouts caused by cascading fault using transmission capacity enhancement of the ac line and load-shedding in the inverter ac system. The salient feature of the proposed scheme is that it provides a very economical way to compensate for the loss of power caused by HVdc line outage. In addition, it does not require communications among the relays. The practical performance and feasibility of the proposed scheme is validated by laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively inhibit the blackouts caused by cascading fault in hybrid ac/dc networks.

Calculation of Ion-Flow Field Near the Crossing of HVdc Transmission Lines Using Equivalent Corona Conductors

The crossing of two-circuit high-voltage direct current (HVdc) overhead lines will happen with the increasing number of HVdc transmission projects. The 3-D calculation is needed to predict the ion-flow field near the crossed HVdc lines. In order to decrease the difficulty of meshing near the bundle conductors, the equivalent single conductors are applied in the modeling. A roughness coefficient converting approach for equivalent conductor in Peek’s formula is presented. Thus, the corona degree of bundle conductors can be reflected by the equivalent single conductor. Afterward, the effectiveness of equivalent corona conductors in ion-flow field calculation is checked by a series of experiments and simulation. Finally, the 3-D ion-flow field distribution near the crossing of ±800 and ±500 kV HVdc overhead lines is calculated using equivalent corona conductors.

A novel pilot directional backup protection scheme based on transient currents for HVDC lines

For HVDC transmission lines, the backup protection cannot respond to internal faults quickly. To solve this problem, a novel pilot directional backup protection scheme is proposed in this paper. Firstly, the fault characteristic of transient currents on both sides of DC bus is analyzed. When forward traveling-wave of current reaches a DC bus, the transient current integration at the back side of the bus over a short time is larger than that at the front side. However, when backward traveling-wave reaches the bus, the current integration at the back side is smaller than that at the front side. Therefore, fault direction can be determined by comparing the transient currents on both sides of DC bus. Then, internal faults can be distinguished from external faults by comparing fault directions at two ends of DC lines, and faulty poles can be selected by comparing the transient currents of two poles. Lastly, to test the performance of the novel protection proposed in this study, a ±800 kV HVDC system is built in PSCAD/EMTDC. Comprehensive test studies show that the proposed protection scheme not only can identify internal faults and select faulty poles correctly and quickly, but also can respond to high resistance ground faults.

A New Protection Principle of Hybrid Bipolar HVDC Line Based on Voltage Fault Component Rate of Change

At present, hybrid HVDC transmission has become a research hotspot. In this paper, the fault-added state characteristics of the hybrid bipolar HVDC transmission system with different faults on the DC transmission line are analyzed by connecting small inductors on both sides of the HVDC transmission line. A single-ended quantity protection method using voltage transient components is obtained. By analyzing the rate of change of voltage fault component, the transient energy component is used to calculate the instantaneous energy magnitude of the transient voltage component to distinguish the fault in the region. The energy function is constructed by the transient voltage of the positive and negative rectifiers to fault selection. The simulation results verify the correctness of the proposed protection principle and can achieve fault pole selection.

Analysis of the Incidence of Direct Lightning over a HVDC Transmission Line through EFD Model

HVDC systems are becoming more common worldwide, specially in Brazil, since the adoption of such system for Itaipu’s hydroelectric complex in the 1980’s. Today, the country has the Xingu-Estreito bipole, with length of 2375 km. This system crosses a region with high lightning incidence, a phenomenon which causes faults in power systems. The most widely used model for the positioning of the arrestor cables over a transmission line is the electrogeometric model. This model, however, does not take into account the different potentials over the structure’s surface, and therefore presents significant inaccuracies when assessing the risk of lightning strikes on structures such as a HVDC line. This work then used the Electric Field Deflection (EFD) model with the aid finite elements. Four levels of lightning are assessed (I, II, III and IV), with current peaks of 3.9, 5.4, 10.1 and 15.7 kA. It was verified that the positive pole tends to attract most of the lightning with shielding failures width (SFW) of 12, 8, 4 and 0 m. It was then proposed to move the arrestor cables horizontally. The study indicates that this horizontal shifting of the cables in 5 and 8 m toward the side with larger chance of direct incidence reduces the shielding failure widths in 50% for peak current of 3.9 kA and almost eliminates the strikes for lightning with peak currents of 5.4, 10.1 and 15.7 kA.

The Subsynchronous Oscillations Caused by an LCC HVDC Line in a Power System Under the Condition of Near Strong Modal Resonance

This paper derives a multi-input multi-output closed-loop model of a power system with an LCC HVDC line. The derived model is comprised of the HVDC subsystem and the subsystem of rest of power system (ROPS). Analysis in the paper indicates that the closeness of open-loop subsynchronous oscillations (SSO) modes of two subsystems on the complex plane is a special condition of near strong modal resonance (NESMOR). Under the condition of NESMOR, strong dynamic interactions are introduced by the LCC HVDC line to degrade the SSO stability of the power system. Theoretical derivation is presented in the paper to estimate the degree of SSO stability degradation caused by the NESMOR. Based on the derivation, a method of open-loop modal analysis is proposed to detect the SSO instability risk brought about by the multivariable dynamic interactions introduced by the LCC HVDC line. An example of multimachine power system with an LCC HVDC line is presented to demonstrate and evaluate analysis and conclusions made in the paper. Dynamic interactions of the LCC HVDC line with a VSC-based HVDC line or a PMSG wind farm occurred in the example power system under the condition of NESMOR, which caused growing SSOs.

Protection of transmission lines in multi-terminal HVDC grids using travelling waves morphological gradient

This paper presents a new method for the protection of transmission lines in voltage source converter based multi-terminal HVDC grids. The fault generated travelling waves at the faulted line ends are filtered by series inductors. The filtered voltage waveform processed by morphological gradient accompanied by fault voltage drop is employed for fast single-end protection of HVDC lines. The proposed method is stable against external DC and AC faults, as well as faults at opposite pole and the breaker opening transients from the adjacent lines. Moreover, it does not require communications between the line ends, it is capable to detect and discriminate relatively high resistance faults, and uses relatively low sampling rate. The effectiveness of the proposed protection method is validated by simulation study on a four-terminal HVDC grid including modular multilevel voltage source converters.

Study on the Effect of Altitude on the Corona Inception Field and the Ground-Level Total Electric Field Under HVdc Lines Using Bipolar Test Lines

With the growing public awareness of environmental protection, the dc ground-level total electric field (TEF) is attracting increasing attention. Especially in high-altitude areas, it has become a key technical factor in deciding conductor types and the height of the transmission line. To determine the altitude effect and correction of the ground-level TEF for HVdc transmission lines, four short bipolar test lines with bundle conductors with the same parameters were built in China at altitudes of 50, 1700, 3400, and 4300 m. Using the test facilities, the dc ground-level TEF was measured over a long period of time, from Jan. 2010 to Nov. 2015. Through a statistical analysis of a large amount of data, the ground-level TEF characteristics of short bipolar test lines at different altitudes were obtained. By comparing the statistical results from different altitudes, it is determined that the dc ground-level TEF and corona inception field strength are linearly dependent on the altitude. Finally, the ground-level TEF and corona inception field strength altitude correction formulas of HVdc transmission lines are proposed.

Calculation and Analysis of Electromagnetic Environment Indexes and Influence Factors of ±1100 kV UHVDC Transmission Lines

In order to study the influence of electromagnetic environment of ±1100 kV Changji-Guquan the UHVDC lines, the ground total electric field and ion current density of the HVDC line is studied in this paper by using the charge simulation method and Deutsch’s hypothesis-based analytical method. And the correctness of the method is verified by the measured results. The EPRI formula and CISPR formula are applied to study the audible noise and radio interference. At last, the influence of UHVDC transmission line parameters on the electromagnetic environment is analyzed, and the influence factors that restrict the construction of the UHVDC transmission lines are also studied. The results show that the minimum height of the pole is limited by the total electric field strength, and the influence of the roughness coefficient should also be taken into account when calculating the minimum height of the wire, while the selection of the diameter of the wire is constrained by both the ground total electric field and the audible noise.

Analysis of Electromagnetic Environment and Influencing Factors of Yihua DC Transmission Lines

The electromagnetic environment of transmission lines is one of the major constraints to the design and construction of HVDC transmission lines. In this paper, the charge simulation method and the analytic method based on the Deutsch hypothesis are used to calculate the total electric field strength and ion current density of the 500 kV Yihua DC transmission lines, and the correctness of the calculation is verified by the actual measurement results. Then the EPRI formula and CISPR formula are applied to analyse the audible noise and radio interference of HVDC transmission lines. At last, the influence of factors such as the pole spacing of HVDC lines, the conductor height to the ground, the splitting spacing and the roughness coefficient on the electromagnetic environment of HVDC transmission lines are analysed. Among them, the height of conductor has effective impact on the total electric field strength, while the control of polar spacing is limited. The height of the conductor and the distance between the poles affect the level of audible noise and radio interference mostly, but the control effect of the two is opposite.

HVDC 계통 내에 초전도 케이블 적용시 선로의 보호 계전에 관한 연구

HVDC 계통 내에 초전도 케이블 적용시 선로의 보호 계전에 관한 연구

Understanding surface degradation on polymeric insulators using rotating wheel and dip test under DC stress

In the present work, surface degradation studies are performed on silicone rubber based polymeric insulators using rotating wheel and dip test under DC voltage application. This work gains importance as number of HVDC lines are being constructed for transferring bulk power over long distances particularly in the country. A newly developed rotating wheel and dip test arrangement is used for the study; provision is made in the experimental set up to apply two different contaminants simulating different environmental conditions simultaneously. In one tank standard contaminant NH 4 Cl as per IEC 62217/IEC TR 62730 is used and in other tank acidic contaminant is used to simulate normal and acidic rain conditions. A comparative study is conducted on the insulator specimen under different environmental stresses to evaluate long term performance. The conductivity of both normal and acidic contaminants is kept 2.5 mS/cm approximately to observe the effect of acidic environment on long term performance. During the experimental duration of 1000 hours leakage current was regularly measured and recorded. Samples were visually investigated and degradation was found to be more under acidic environment and tracking was observed. After the experiment, physico-chemical analysis is conducted on the degraded samples. Fourier transform infrared (FTIR) spectroscopy is utilized to analyze the surface chemical changes on degraded insulators samples. Salt deposition was detected on aged samples in the FTIR spectrum. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Analysis (EDAX) were conducted to detect elemental presence over the surface of degraded/aged samples. Thermo-Gravimetric Analysis (TGA) is utilized to analyze thermal stability and loss of ATH fillers before and after aging experiment.

Sub‐synchronous oscillations caused by the control of LCC HVDC transmission line under the condition of open‐loop modal coupling

This study examines the sub-synchronous oscillations (SSOs) caused by the control of a line commutated converter high-voltage, direct current (LCC HVDC) transmission line in a power system with a doubly fed induction generator for the wind power generation. The examination is based on a closed-loop interconnected model consisted of an open-loop HVDC subsystem and an open-loop subsystem of power generation. The open-loop modal coupling is the modal condition that an SSO mode of the open-loop HVDC subsystem is close to an open-loop SSO mode of the power generation subsystem on the complex plane. Analysis in the study indicates that under the condition of open-loop modal coupling, the LCC HVDC line may induce strong dynamic interactions which degrade the damping of power system SSOs. An SSO index is derived to assess the degree of damping degradation of the SSOs, which can be used to identify the SSO instability risk brought about by the control of LCC HVDC transmission line. Hence, the mechanism of why the control of LCC HVDC transmission line may cause power system SSOs is revealed from the perspective of open-loop modal coupling. Study cases are presented to demonstrate and validate the analysis and conclusions made in the study.

Analysis and Measures of Ultralow-Frequency Oscillations in a Large-Scale Hydropower Transmission System

The Yunnan power grid has a large-scale hydropower station, whereby the power generated at the station is transmitted via HVdc lines under an asynchronous operation mode. In the system performance test of the Yunnan asynchronous operation, a 0.05-Hz ultralow-frequency oscillation occurred in the grid. This ultralow-frequency oscillation is a frequency stability problem caused by instability of hydraulic turbine governing system, which is quite common in the islanded system and very rare in the normal synchronous system. The relationship between the speed governing system stability of one unit and that of the system with parallel operation units was proposed to explain that the high proportion of units with an unstable governing system will lead to system frequency instability and the reduction in loads in the traditional sense was proven to further worsen the frequency stability of the Yunnan asynchronous system. By using related measures, the ultralow-frequency oscillation of the Yunnan asynchronous system has been curbed effectively. System performance testing verified the effectiveness of the analysis and the measures.

The Asymmetric Alternate Arm Converter: A Compact Voltage Source Converter Design for HVDC

The Modular Multilevel Converter (MMC) has become one of the most practical topologies for integration of renewable energy power plants into the grid, through submarine HVDC cables and HVDC lines. Nonetheless, the MMC requires significant large DC storage capacitors to mitigate for large power, low frequency pulsations due to the single-phase like AC to DC conversion process in their arms. This work addresses the recently introduced Asymmetric Alternate Arm Converter, a hybride MMC topology which uses modular converter arms only on the lower side of the main converter whereas the upper side uses only standard ON-OFF valves topology. This converter is similar to the Alternate Arm Converter (AAC) but dispenses of the modules on the upper side and of the director switch on the lower side. Like the AAC, the Asymmetric AAC offers a significant reduction in DC energy storage requirements and in the number of modules. In addition, the Asymmetric AAC offers a less restricted operation than the AAC, in terms of operating over a wide range of AC to DC voltage ratio, and the possibility of achieving both distortion-free AC current and ripple-free DC current. The principle of operation of the Asymmetric AAC is revisited here and a suitable control strategy to demonstrate its operation is developed. In addition, requirements for energy storage, voltage and current rating of converter modules and converter losses are examined by means of simplified analyses. Proposals are verified through simulation studies and experiments conducted on a low power prototype having three H-bridge modules per arm. Results confirm good operation of the proposed converter and control scheme.

Subsynchronous Oscillations Caused by Open-Loop Modal Coupling Between VSC-Based HVDC Line and Power System

A closed-loop interconnected model of a power system with a voltage source converter (VSC)-based HVDC line (VSC-HVDC line) is established, wherein, a selected control system of the VSC-HVDC line and rest of the power system are modeled as two separate open-loop subsystems. Based on the established model, a special condition of open-loop modal coupling is studied, when a complex pole of the open-loop selected control subsystem of the VSC-HVDC line is close to a subsynchronous oscillation (SSO) mode of open-loop power subsystem on the complex plane. Theoretical analysis indicates that when the open-loop modal coupling occurs, subsynchronous interactions (SSIs) between the VSC-HVDC line and the rest of power system may become strong. It is very likely that the open-loop modal coupling may degrade the damping of power system SSOs. Hence, the mechanism that the VSC-HVDC line may cause the SSOs in the power system is revealed from the perspective of open-loop modal coupling. In this paper, study cases of example power systems with the VSC-HVDC line are presented to demonstrate and evaluate the analysis and conclusions made.

Optimal placement and control variable setting of power flow controllers in multi-terminal HVDC grids for enhancing static security

This research proposes an approach to select an optimal place and control variable setting for recently proposed Power Flow Controller (PFC)s including series, cascaded, and interline PFCs in Multi-Terminal High-Voltage DC (MT-HVDC) grids based on sensitivity analysis technique to enhance static security. To do so, appropriate static power injection models of the PFCs are obtained. Accordingly, sensitivity relationships between the defined power performance index and each PFCs control variable are obtained. It is for the optimal PFCs placement purpose in order to utilize maximum capacities of the transmission grid. Optimal settings for the PFC as well as the controllable voltage source converters are computed in turn by applying sequential quadratic programming solver to the developed security-based DC optimal power-flow problem which includes several corrective constraints. The study scope is single contingencies affecting HVDC lines with the objective of eliminating the consequent overloaded HVDC lines and thereby enhancing the MT-HVDC grid security by controlling the power flows within the MT-HVDC grid. Static simulations are performed considering two generic four-terminal and eighth-terminal MT-HVDC test grids in order to demonstrate the proposed methods effectiveness and authenticate its robustness. The obtained results indicate that the MT-HVDC grid can remain secure under single HVDC line contingencies in most cases by implementing the proposed method.

A novel distance protection scheme for HVDC lines based on R-L model

HVDC transmission system is becoming increasingly desirable for its obvious technical and environmental advantages. A novel single-end distance protection scheme for HVDC lines is proposed in this paper. It is based on the linear distribution of low frequency voltage signals between the relay point and setting point. An internal fault can be detected if the low-pass filtered measure voltage is opposite in sign with the compensating voltage at the setting point, which can be calculated with the R-L differential-equation algorithm. To find a tradeoff between operating speed and accuracy, the protection is divided into K zones, each of which has different protection scopes and cut-off frequencies. A two-terminal HVDC system and a multi-infeed HVDC system are respectively built in EMTDC. Comprehensive simulation results have indicated that the proposed protection scheme has satisfactory operation performance and will be not influenced by system topologies or parameters. Besides, it is not sensitive to noises or voltage levels. Compared with many other single-end protections for HVDC lines, this novel distance protection has clear setting principles, lower sampling frequency, smaller calculation amount and faster operating speed.

A Fast Non-Unit Protection Method Based on S-Transform for HVDC Line

To overcome the problems existing in the main protection of HVDC transmission lines, such as low reliability and poor ability of resisting high ground resistance fault, this paper proposes a novel non-unit protection method based on S transform. Analysis shows the high and low frequency components of the transient voltage signals suffer from different degrees of attenuation when they pass through the HVDC line boundary. Therefore, according to this characteristic, the voltage signals of specific frequency bands are extracted accurately using S transform, which be used to identify the internal from external faults. By comparing the high and low frequency components generated by the lightning disturbance with those generated by the faults, S transform based energy ratio criterion to identify lightning disturbance is constructed. In addition, for a bipolar HVDC transmission system, a method using the 0 Hz energy difference of S transform of the voltage transient signals is proposed to identify the faulted pole. Simulation results verify the feasibility and validity of the proposed protection method.

Fault Handling Methods and Comparison for Different DC Breaker Topologies and MMC Topologies of the HVDC System

The HVDC system has many significant benefits and is widely used around the world. The protection of HVDC system is always an issue, which should be solved. This paper presents the working principle of different protection schemes. The advantages and disadvantages of these protection schemes are also introduced. In order to solve the HVDC fault, two protection strategies are proposed. One design focusses on the topologies of the HVDC breaker in the HVDC line, such as all-solid HVDC breaker, resonant HVDC breaker, and hybrid HVDC breaker. The other design is from the viewpoint of converter topology, which has two types. One type generates the counter-emf in arms, such as full bridge MMC, hybrid MMC, and clamp-double MMC. The other type cuts off the current path from the AC side to the DC side, which is also introduced in this paper. Some performances of these topologies are compared, such as switching time and efficiency.