High-voltage Direct Current Transmission Lines Research Articles

Performance analysis of silicone rubber insulator in DC high-voltage inclined plane tracking test

The present trend in electric power transmission technology evolves with the latest high-voltage direct current (HVDC) transmission network. Silicone rubber (SiR) composite insulators are predominantly used since two decades for high-voltage alternating current (HVAC) transmission lines. In case of HVAC transmission line, the performance of SiR insulators is promising; however, there are failures reported with their uses for HVDC transmission line. In this research work, SiR composite formulations are varied to study their effective changes in the direct current (DC) tracking and erosion resistance. Four different weight percentage (wt%) aluminum trihydrate (ATH) fillers loaded SiR formulations are prepared and invariably subjected to positive polarity DC inclined plane tracking (DCIPT) test before and after ultraviolet (UV) aging. The Fourier transform infrared spectroscopy study has been carried out to understand their morphological changes after and before UV aging. The results of these tests indicated that DC tracking and erosion resistance effectively reduced severe surface degradation for UV-aged SiR material. However, the 40 wt% ATH-filled SiR sample performed well exhibiting higher time to failure in positive DCIPT and thus could be better usable as HVDC insulation material.

Study on Emergency Load Shedding of Hybrid AC/DC Receiving-End Power Grid with Stochastic, Static Characteristics-Dependent Load Model

When large-capacity HVDC (high voltage direct current) transmission line blocking occurs in a hybrid AC/DC (alternating current/direct current) power grid, the receiving-end system will encounter a huge power imbalance, which will lead to a frequency drop and redistribution of the power flow, and which may further lead to the overload of other transmission lines, cascading failures and a large-scale blackout. To resolve these problems, an emergency load-shedding strategy for the DC receiving-end system is proposed from the perspective of a quasi-steady state. The proposed method can accurately calculate the actual total power imbalance by modeling more detailed stochastic loads with static frequency/voltage characteristics and involving the inertia effect of the generator during the response delay period, which can effectively reduce the amount of load curtailment. In addition, several factors affecting the power imbalance estimation in stochastic scenarios and their mechanisms are analyzed in detail, and the key aspects relevant to the DC blocking fault analysis are identified as well. Finally, the influence of different load-shedding strategies on the receiving-end system security after a DC blocking fault is compared with the security indices, including those that are relevant to the frequency/load change proposed herein, and a uniform load-shedding coefficient is obtained via the proposed method, even for different power imbalances under a stochastic context, which makes the load-shedding strategy more practical.

Control strategies to improve stability of LCC‐HVDC systems with multiple MMC taps

High-voltage direct current (HVDC) is a proven technology for grid integration of renewable energy sources such as offshore wind farms and interconnecting distributed systems to main power grids. One economical solution for electrifying rural communities is to extract a small amount of power from existing HVDC transmission lines with power electronic converters, which is called tapping. This study analyses the feasibility and system performance of parallel tapping line-commutated converter (LCC)-HVDC systems with multiple full-bridge modular multilevel converters under various fault scenarios and operating conditions. Simulation results reveal that undesirable system disturbances such as DC-link voltage sags, DC-link current overshoots, and the transient reduction of inverter extinction angle are imposed on the LCC-HVDC system in the case of tap station AC side faults. Such disturbances would endanger the reliable operation of the entire LCC-HVDC system with the parallel taps. Furthermore, this study proposes two fault mitigating schemes, i.e. a tap station current modulation controller and three supplementary controller configurations, to reduce the impact of tap station AC side faults on the LCC-HVDC system. Both fault mitigating schemes are controller-based solutions, which are augmented to the existing controllers by utilising only local measurements. The proposed schemes are verified through simulations in PSCAD/EMTDC.

A Method for the Indirect Detection of Audible Noise From High-Voltage Direct Current Transmission Lines

Corona discharge will occur on high-voltage direct current (HVDC) transmission lines with the increase in voltage level, accompanied by audible noise, radio interference, and other phenomena. Audible noise has a serious negative impact on the lives and health of the residents around transmission lines. Therefore, it is very important to detect and study audible noise for the optimal design of transmission lines. However, the existing audible noise detection method is the direct detection method, which is vulnerable to the background noises. It is difficult to obtain the accurate measurement values in complex outdoor environment. Based on the relationship between audible noise and corona current, this article presents a method to obtain the A sound level of audible noise indirectly by measuring the corona current, which is called the indirect detection method of audible noise. The validity and accuracy of the proposed method are verified by the experiments of outdoor corona cage and actual HVDC transmission line segment. The proposed method provides a new way to solve the problems of direct detection of audible noise and can complement the existing method.

Control and stability analysis of VSC-HVDC based transmission system connected to offshore wind farm

Offshore wind farms (WFs) with significant capacities have been installed recently all over the world. In order to transmit the WF power to the onshore grid, high voltage direct current (HVDC) transmission system is appropriate technology. This paper analytically studies the impact of system parameters, controllers and operating conditions on the dynamic behavior of HVDC transmission systems based on three-level neutral point clamped voltage source converters (VSC). Also, it investigates modeling, control and stability analysis of VSC-HVDC system connected to the offshore wind farms. The VSC-HVDC system comprises offshore and onshore converters and high voltage dc transmission lines. The paper extracts VSC-HVDC system dynamics at the dc-side and argues the interaction between onshore converter control and HVDC transmission line dynamics. Moreover, the paper presents controller design for the dc-link voltage regulation by the onshore converter and examines the impacts of HVDC line length and dc voltage control bandwidth (BW) on the system stability by the modal analysis and time domain simulations.

Long-term evaluation of HVDC transmission line audible noise and its correlation with background noise

The audible noise (AN) generated by corona discharge from a full-scale, high-voltage direct-current (HVDC) transmission line was evaluated over a one-year period from December 2017 to November 2018. The double bipolar HVDC transmission line using a metallic return conductor was demonstrated using a full-scale tower having a height of 21 m in Gochang, Korea. The measured AN in an outdoor environment was statistically evaluated and compared in different seasons and weather conditions. The A-weighted spectrum of the HVDC corona noise exhibited a greater distribution than the background noise above 1.0 kHz in the frequency domain. Various background noises were considered and eliminated to improve the measurement accuracy. The highest AN levels were exhibited during the spring season. Individuals in the age range of 18–60 years were randomly selected to conduct a human-perception test. The results indicated that 15% of the participants became aware of the AN from the corona discharge when it was >5 dBA greater than the background noise. The AN was <36 dBA when the conductor surface gradient was <18 kV/cm with a six-bundle sub-conductor.

A novel non-unit transient based boundary protection for HVDC transmission lines using synchrosqueezing wavelet transform

Since high voltage direct current (HVDC) transmission system takes on significant tasks in coordinating the power grids asynchronous interconnection and the comprehensive utilization of energy resources, the reliable and fast faults clearance to guarantee the safety and stability of the power system should be attached great attention. This paper, based on the analysis of the HVDC boundary characteristics, proposes a novel non-unit transient based boundary protection as the main protection for HVDC transmission lines, and uses the novel synchrosqueezing wavelet transform (SWT) for time-frequency (T-F) representation. By synchrosqueezing and reassigning the T-F spectrum of the continuous wavelet transform (CWT), SWT can obtain a high resolution T-F spectrum, which brings higher reliability to protection. Based on the transmission differences between tuning voltage and low-frequency voltage by the boundary, this paper proposes the SWT-based non-unit transient boundary protection algorithm, and the ratio of the positive and negative voltage micro-increment is used to select the faulty line. Extensive PSCAD simulations of ±800 kV HVDC project and some field data are used to test the proposed protection scheme. Compared with the CWT-based boundary protection, experimental results verified the proposed method could perform rapidly and reliably in various types of fault, especially can be immune to commutation failure and shows good performance for switching operation as well as fault closer to the very begging of the line, and with high selectivity and sensitivity.

Optimization of Radio Interference Levels for 500 and 600 kV Bipolar HVDC Transmission Lines

In this work, a method to compute the radio interference (RI) lateral profiles generated by corona discharge in high voltage direct current (HVDC) transmission lines is presented. The method is based on a transmission line model that considers the skin effect, through the concept of complex penetration depth, in the conductors and in the ground plane. The attenuation constants are determined from the line parameters and the bipolar system is decoupled by using modal decomposition theory. As application cases, ±500 and ±600 kV bipolar transmission lines were analyzed. Afterwards, parametric sweeps of five variables that affect the RI levels are presented. Both the RI and the maximum electric field were calculated as a function of sub-conductor radius, bundle spacing, and the number of sub-conductors in the bundle. Additionally, the RI levels were also calculated as a function of the soil resistivity, and the RIV (radio interference voltage) frequency. Following this, vector optimization was applied to minimize the RI levels produced by the HVDC lines and differences between the designs with nominal and optimal values are discussed.

Frequency control of power system including PV and wind farms by using output frequency band control of HVDC interconnection line

Due to serious environmental issues in recent years, renewable energy has been focused on all over the world to solve the issues. However, because of indirect characteristics such as variations of solar radiation intensity or wind speeds, the output of power generation utilizing renewable energy becomes unstable. As a result, frequency and voltage in the power system can fluctuate. To solve this problem, various methods to control the frequency variations have been studied and reported until now, one of which is based on High voltage direct current (HVDC) transmission line. This paper proposes a novel control scheme to reduce the frequency variations due to large penetrations of wind power and photovoltaic (PV) power, which is introduced to power flow control of HVDC transmission line with utilizing a notch-band pass filter in its frequency control system.

Calculation of Accumulated Charge on Surface of Insulated House Near DC Transmission Lines by Method of Characteristics

Charges resulting from corona on dc transmission lines will be driven by the electric field to the space and form the ion flow field. Some space charges will accumulate on the surface of the insulated buildings in the vicinity of high-voltage direct current transmission lines and may lead to human body shock. An improved iterative method is used to solve the ion flow field and accumulated charges according to the boundary condition that the normal component of electric field on the insulated building surface is zero. The accumulated charges are calculated by a 3-D method of characteristics based on the known ion flow field, which avoids large-scale numerical calculation. The calculation results are in a good agreement with the measured data of reduced-scale test under laboratory conditions.

Analysis of the relationship between DC component and spectral components of corona current on HVDC transmission lines

Corona current is an important parameter to characterise the corona discharge on high-voltage direct current (HVDC) transmission lines. The spectral components of corona current are closely related to a variety of corona effects, such as audible noise. It is of great significance to study the frequency-domain characteristics of corona current. Here, the relationship between DC component and spectral components of corona current on HVDC transmission lines in the band of 2–20 kHz was analysed. First, a lot of corona current data were measured in two HVDC experiment bases. Then, the expression representing the correlation between DC component and spectral components of corona current within 2–20 kHz was established with the maximum surface nominal electric field intensity of conductor as an intermediate variable. Finally, the correctness and universality of the relationship found were verified. The research results here not only reveal the characteristics of corona discharge but can also provide a new way for obtaining the spectral components of corona current in 2–20 kHz with its DC component, and can lay the foundation to provide a new way to get audible noise indirectly based on the DC component of corona current in the future.

Non-Contact Voltage Monitoring of HVDC Transmission Lines Based on Electromagnetic Fields

The high-voltage direct current (HVDC) transmission network is envisioned to advance further as a supplement to AC transmission in a new era of renewable energy and smart grid development. Voltage monitoring of HVDC transmission lines is critical for evaluating the power quality in power delivery and realizing the stability control of the power system. Unfortunately, the traditional potential transformers can only work with AC voltage while the other existing voltage sensors (e.g., Hall-effect voltage sensors, and optical-fiber voltage sensors) are still not practical for large-scale deployment for monitoring the HVDC transmission lines, owing to the fact that they are either invasive or expensive. In this paper, a non-contact electric-coupling-based voltage sensing with the assistance of magnetic-field sensing for monitoring the voltages of HVDC transmission lines is proposed. The voltages of HVDC transmission lines can be reconstructed from measuring the induced voltage of induction bars, and their correlation coefficients are attained by the magnetic sensing with an optimization program which determines their relative spatial positions. The sensitivity and stability of the proposed platform, and the corona effect of the HVDC transmission lines are discussed. The performance of the proposed sensing technique was investigated by simulation on a ±100 kV HVDC transmission line system, and was further experimentally validated on a scaled HVDC transmission-line testbed in the lab. This sensing technique can be implemented at low cost by adopting copper induction bars and magnetoresistive sensors. Considering its non-contact nature and cost-effectiveness, this technique is suitable for a large-scale deployment over HVDC transmission networks for wide-area monitoring.

Novel protection scheme for high‐voltage direct‐current transmission lines based on one‐terminal transient AC voltage

This paper considers the distributed capacitance of transmission lines to research the protection of high-voltage direct-current (HVDC) transmission line. According to the superposition theorem, the post-fault superimposed network of HVDC system is used to analyse the characteristic of the root mean square (RMS) of specific frequency voltage (SFV) during the transient period of internal and external faults. At the parallel resonant frequency of DC filter, for internal faults, the fluctuation of the RMS of transient AC voltage is larger than that of external faults. Based on the fluctuation characteristic of single-terminal transient AC voltage, a novel protection scheme for HVDC transmission lines is proposed. Various simulation results show that the proposed protection is immune to fault resistance and performs well under different fault types.

Unbalanced current analysis and novel differential protection for HVDC transmission lines based on the distributed parameter model

The current differential protection presently being used for high-voltage direct-current (HVDC) transmission lines requires increases in time delay and threshold to avoid misoperation, which is induced from a distributed capacitance current along the transmission line, leading to low operation speed and disabling the device from distinguishing between internal and external faults. The unbalanced currents of the non-fault pole are studied in two cases: under an external fault as induced by the distributed characteristics and under a single-pole-to-ground fault as induced by the coupling characteristics of the transmission line parameter. To overcome its defects, this paper develops a new differential protection algorithm based on the distributed-parameter transmission-line model. The fault identification criterion using the compensated current of the common mode and fault pole selection criterion using the compensated current of the differential mode are introduced. Unaffected by the distributed capacitance current, this differential protection correctly operates with high sensitivity through the entire process during the fault. The feasibility and validity of the differential protection algorithm are verified.

Analysis of the influence of dynamic components of AC/DC system on transient voltage stability based on energy function method

With the interconnection of large power networks, transient voltage stability of AC/DC power systems has been paid more attention in recent years. To investigate main influence of generating facility, high-voltage direct current (HVDC) and motor load for transient voltage stability, from the perspective of the generalised branch potential energy, the new indexes, and analysis method is proposed here. First, an energy function for AC/DC system is constructed with major dynamic components, namely classical HVDC control systems and motors in third-order model. Accordingly, the influence effects of the above major dynamic elements are investigated based on indexes from the corresponding generalised branch potential energy. With the rule of change in transient potential energy distribution in the network and information in stability margins, an evaluating procedure is established. Here, the judgment of transient voltage stability employs a heuristic energy function method. Finally, simulation tests on three-machine system with a HVDC transmission line are conducted in PSAT package, and the obtained results demonstrate the feasibility and effectiveness of the proposed method.

Calibration of a sensor for an ion electric field under HVDC transmission lines

With the development of high-voltage direct-current (HVDC) transmission projects, the issue of electromagnetic compatibility has become more and more severe, especially the synthetic ion flow electric field. The DC electric field is inseparable from the equipment insulation, the safety of the system operation, and effects on organisms nearby. The measurement of the synthetic electric field of ion flow has been a major difficulty due to the influence of ion currents and distorted field. A differential electric field sensor for measuring the synthetic electric field is proposed. The calibration method and the experimental device for the measurement of the air synthetic electric field of ion flow are presented. Calibration experiments show that the calibration method is practical and effective, and the test result from the sensor is in good agreement with the theoretical value. The experiments in the library show the validity of the calibration method and the excellent performance of the sensor.

Transient numerical calculation and differential protection algorithm for HVDC transmission lines based on a frequency-dependent parameter model

Being free from the influences of the transmission characteristics, the current and voltage near a fault point can better reflect the fault features. Thus, these electrical quantities along a transmission line can be used to formulate the protection criteria to improve the sensitivity and selectivity of the protection. A numerical calculation method for transient electrical quantities along a transmission line based on a frequency-dependent parameter transmission line model is studied. Differential protection uses currents from both terminals and can protect transmission lines with excellent selectivity. However, the differential protection currently used for high-voltage direct-current (HVDC) transmission lines requires an increase in the time delay and the threshold to avoid mis-operation induced by the distributed capacitance current along the transmission lines, which results in low operation speeds and poor sensitivity. This paper presents a new differential protection algorithm using the calculated current at compensation points to overcome this problem. Because the parameters involved in the differential protection can be precalculated offline, this algorithm has low computational complexity and can be easily implemented. Unaffected by the distributed, frequency-dependent, and coupling characteristics of the transmission line parameters, the proposed differential protection approach can operate correctly with high sensitivity throughout the entire process under fault. Simulations demonstrate the feasibility and validity of the differential protection algorithm.

Optimization Model for the Short-Term Operation of Hydropower Plants Transmitting Power to Multiple Power Grids via HVDC Transmission Lines

Long-distance and large-capacity hydropower transmission from large hydropower plants in southwestern China to load centers is an important and effective measure for the accommodation of large-scale hydropower in China. However, the load demands of the receiving-end power grids are not fully considered in the conventional power transmission mode, which has greatly affected their enthusiasm for the absorption of trans-regional hydro energy. In order to make full use of the peak shaving capability of the hydropower plants, this paper develops an optimization model for determining the hourly generation scheduling of the hydropower plants transmitting electric power to several power grids via high voltage direct current (HVDC) transmission lines. A large-capacity and highly complex multi-unit hydropower system, the Xiluodu plant in China, is taken as the case study. In the proposed model, minimizing the peak-valley differences of multiple receiving-end power grids is adopted as the objective to alleviate the peak shaving pressure of the receiving-end power grids. In addition to the traditional hydraulic constraints, the operation constraints of individual units and HVDC power transmission limits are well considered. The study focuses mainly on modeling the stair-like power transmission curve constraint which is discrete and nonlinear and has been rarely considered in previous studies. This constraint is then linearized through limiting the logical relations between multiple binary integer variables. Case studies demonstrate that the proposed model has high computational efficiency, and the peak-valley differences of the two receiving-end power grids, i.e., Zhejiang Power Grid and Guangdong Power Grid, are decreased by 15.2% and 7.33%, respectively. Moreover, frequent conversion of HVDC converter equipment can also be avoided, which makes the obtained generation schedule more executable.

WSN-Based Measurement of Ion-Current Density Under High-Voltage Direct Current Transmission Lines

Ion-current density is a significant indicator of electromagnetic environment under high voltage direct current lines. To improve measurement accuracy and convenience, this paper concluded factors that affect measurement results and a wireless measurement method was proposed. Wilson plate kind of sensor was designed according to the IEEE standard. The ion current was measured and digitalized by the measurement device, and the signal was transmitted to the computer via wireless sensor networks. The data were analyzed and recorded by automatic measurement software. Finally, the measurement system was calibrated and some field tests had been performed, the results show that the measurement system possesses the merits of good accuracy, simple structure, convenient operation, and high stability and reliability. The maximum current measurement error is less than 3%.

Analysis of Transient Behavior of Mixed High Voltage DC Transmission Line Under Lightning Strikes

Growing urbanization coupled with increased power demands have led to increasing use of mixed power transmission lines with sections of overhead lines (OHL) and underground cables. Due to differences in surge impedance of cables and OHL, voltage surges experience reflections and refractions at their boundaries which make the transient behavior of mixed high voltage direct current (HVDC) line quite peculiar. Lightning strikes on overhead sections of lines induce voltage surges that travel along OHL and enter the cable section. Lightning overvoltage can cause OHL insulators to flashover and stress cable insulation or cause its permanent breakdown. In this paper, we simulated a fast front model of a mixed HVDC transmission line using an electromagnetic transient simulation program (PSCAD) to analyze its transient behavior under a lightning strike. The leader progression model has been used to predict the dielectric performance of OHL insulators. It has been shown that transition towers adjacent to the cable section are much less vulnerable to flashover than subsequent towers. The length of a riser section (connecting OHL and cable) and tower footing impedance have shown to significantly influence the flashover performance of OHL insulators. In addition, the length of cable segments and sheath grounding impedance has been found to influence cable overvoltage. This paper can be used to evaluate the insulation coordination and overvoltage protection requirements for a mixed HVDC transmission line.