Direct Current Transmission Lines Research Articles

Travelling wave fault location principle for hybrid multi-terminal LCC-VSC-HVDC transmission line based on R-ECT

This paper investigates the fault location issue of hybrid multi-terminal high voltage direct current (HVDC) overhead transmission line based on the line commutated converter (LCC) and voltage source converter (VSC). There exists two issues that should be addressed: (i) the travelling wave (TW) fault location principle’s adaptability to multi-terminal network topology and converter structures; (ii) the transfer method of high frequency TW signal. Therefore, a novel TW fault location principle for the hybrid multi-terminal LCC-MMC-HVDC transmission line is proposed based on the Rogowski coli based electronic current transformer (R-ECT). Firstly, the high frequency TW transfer capability of the R-ECT is studied. Secondly, a novel TW fault location principle based on the secondary differential current travelling wave (D-CTW) is proposed. Eventually, in order to verify the high accuracy of the proposed principle, a typical ±400 kV hybrid three-terminal LCC-VSC-HVDC transmission system is used for the case study based on PSCAD/EMTDC. Furthermore, its robustness against different fault locations, different fault resistances and different fault types is also investigated.

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.

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.

Enhancing Nigeria’s power grid performance through a hybrid transmission network

The Nigerian power demand is increasing and the existing power grid has proved over-time to be ineffective in meeting up with the increase. The transmission losses experienced on the grid account for a greater part of its problem. The hybrid setting (that is, integrating High Voltage Direct Current (HVDC) transmission lines) was proposed in this work. In the hybrid system, six existing transmission lines representing twenty-nine per cent (29%) of the total lengths of the transmission lines on the existing grid were converted to HVDC lines. Converter stations were introduced at the sending and receiving ends of the lines in the hybrid model. The modelling and simulation of the existing grid was done on Matlab/Simulink while that of the hybrid system was also carried out. The load flow results showed the voltage and load consumption profiles. The total loss on the existing network was 438.1MW while that of the hybrid just added up to 104 MW, the efficiency recorded were 91% and 97% respectively. The results presented the hybrid setting as a better option as improved voltage level was experienced and the losses were drastically reduced.Keywords: Grid, High Voltage Direct Current, Hybrid, Simulink, Transmission losses

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.

Study on the Corona Discharge Ionized Field of UHVdc Based on Particle-in-Cell Iterative Method

In this paper, an accurate time-domain model was established to calculate the ion-flow field of ultrahigh-voltage direct current transmission line. First, the influence of corona layer thickness is considered in the model. Second, the ionic-flow field is calculated without using the traditional fluid model in which the mobility is set as a constant but based on the particle-in-cell iterative method, and the migration of charged particles is analyzed which considering different forces in motion. Therefore, the nonlinear migration of the electric charge can be considered by this calculation process, making the model closer to the actual situation. Third, coaxial cylinder model is used to verify this method. Finally, corresponding calculation results of −1100 kV transmission line were obtained by this method, and the impact of wind speed is also studied in this paper.

Power shift‐based optimal corrective control for bipolar multi‐terminal high‐voltage direct current grid

The bipolar modular multi-level converter-based multi-terminal high voltage direct current (MTDC) grid offers an attractive solution to cope with the increased renewable power generation integration and power transfer capability. However, the N − 1 outage of direct current transmission lines and converters may result in overloads of other components and renewable power generation congestion. This study presents a power shift-based optimal corrective control method to guarantee the operational security of the system with bipolar MTDC grid embedded. First, the principle of power shift in the bipolar MTDC grid is analysed. The grid features in isolated positive and negative poles and can provide up to 50% transmission redundancy. Following an N − 1 contingency, certain power can be shifted from the faulty pole to the healthy pole or between different converter stations. Second, two optimisation models are, respectively, established for the MTDC and alternating current grids to address the power shift-based optimal corrective control. The proposed method guarantees operational security of the system while the amount of renewable power generation accommodation by the MTDC grid is maximised. Finally, the method is applied to the real-life Zhangbei ±500 kV four-terminal bipolar high-voltage direct current grid. Numerical results and electromagnetic transient simulation verify the effectiveness of the presented method.

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.

An Optimized Coordination Strategy between Line Main Protection and Hybrid DC Breakers for VSC-Based DC Grids Using Overhead Transmission Lines

Compared with alternating current (AC) power grids, the voltage-sourced converter (VSC)-based direct current (DC) grid is a system characterized by “low damping”, as a result, once there is a short-circuit fault on the DC transmission line, the fault current will rise more sharply and the influence range will be much wider within the same time scale. Moreover the phenomenon that a local fault causes a whole power grid outage is more likely to occur. Overhead transmission lines (OHLs) have been regarded as the mainstream form of power transmission in future high-voltage, large-capacity and long-distance VSC-based DC grids. However, the application of overhead transmission lines will inevitably lead to a great increase in the probability of DC line failure. Therefore, research on how to isolate the DC fault line quickly is of great significance. Based on the technology route for fault line isolation using DC breakers, on the basis of in-depth analysis of traditional coordination strategy, an optimized coordination strategy between line main protection and a hybrid DC breaker for VSC-based DC grids using overhead transmission lines is proposed in this paper, which takes the start-up output signal of line main protection as the pre-operation instruction of the corresponding hybrid DC breaker. As a result, the risks of blockage of the modular multilevel converter (MMC) closer to the fault position and of damage to power electronic devices in main equipment can be reduced effectively. Finally, the proposed coordination strategy was verified and analyzed through simulation.

A method for fault section identification in High voltage direct current transmission lines using one End measurements

To identify the section of fault in line commutated converter based high voltage direct current transmission lines, a new technique has been proposed in this work. The proposed technique depends on extraction of rectifier end current signal of the transmission line. Three separate fuzzy inference system modules have been proposed to carry out the protection tasks. First fuzzy module detects the faults both in AC and DC section. The second fuzzy module discriminates the section of the fault. The third fuzzy module identifies fault pole and classifies it. The advantage of the proposed method include 100% reach setting, fault detection time is less than one cycle, require no communication links. Besides, the protection scheme covers and discriminates the grounding fault with fault resistance up to 500Ω. With all the above mentioned advantages, the proposed fuzzy based method can be used efficiently for fault section identification in HVDC transmission lines.

Wavelet‐based EMTR method for fault location of VSC‐HVDC transmission lines

A wavelet-based electromagnetic time reversal method for locating faults in voltage source converter-high-voltage direct current (VSC-HVDC) transmission lines is proposed in this study. This principle, firstly, is to use wavelet decomposition to filter the 1-mode fault currents, which are recorded at ends of the transmission line. Secondly, the filtered currents are mirrored on time axes, which called time reversal. Thirdly, the time-reversed currents are set as current sources at ends of the lossless mirror lines. Through assuming a fault at each point of line, the assumed fault currents are calculated, and the RMS of fault current at actual fault location is maximal according to a strict scientific proof. Theoretically, the Wavelet-based Electromagnetic Time Reversal (WEMTR) method is robust against fault types and resistance. A multilevel converter-HVDC system is established in Power Systems Computer-Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC) to verify the feasibility of the proposed method, and the transmission line is simulated by frequency independent phase model. The results show that, the fault location can be calculated exactly without high sampling rates.

A Novel Ultra-High-Speed Traveling-Wave Protection Principle for VSC-based DC Grids

Protection of direct current (DC) transmission lines is one of the key difficulties to be urgently solved in the construction of the future voltage-sourced converter (VSC)-based DC grids. In this paper, a novel ultra-high-speed traveling-wave (TW) protection principle for DC transmission lines is proposed which is based on characteristics of modulus voltage TWs. First, the absolute value of the change in amplitude of the 1-mode voltage TW is used to construct the protection starting-up element. Then, the dyadic wavelet transform is utilized to extract the wavelet-transform modulus maxima (WTMM) of 1-mode and 0-mode initial reverse voltage TWs separately, which are used for fault section identification and selection of fault line successively. A four-terminal annular VSC-based DC grid electromagnetic transient model is established in PSCAD/EMTDC, and performance of the proposed novel ultra-high-speed TW protection principle is evaluated. Extensive simulation results under different fault conditions show that the proposed ultra-high-speed TW protection principle is excellent in rapidity, reliability and robustness. Therefore, the proposed novel ultra-high-speed TW protection principle in this paper can be adopted as an outstanding main protection for VSC-based DC grids.

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.

Comparative Analysis of Electrical Power Utilization in Nigeria: From Conventional Grid to Renewable Energy-based Mini-grid Systems

The electrical power grid in Nigeria is limited in terms of reach and utilization, and this leaves a large section of the population without access to power supply. This paper therefore reviews literature on the current state of the conventional electrical power grid in Nigeria. The generation, transmission and distribution sectors of the grid are briefly reviewed before examining the extent, capacity and power generation technologies used by some currently deployed mini-grid systems. It is observed that a majority of deployed mini-grid systems depend on solar photovoltaic renewable energy sources and such systems are mostly isolated from the conventional grid. Therefore, to make a case for improved access to electrical power supply in the country, statistical and demographic analysis has been carried out to reveal the size of the electrical energy market available to mini-grids and the need to integrate current and future mini-grids to the conventional grid. It is noted that with substantial but targeted investments, a number of existing smart grid technologies can be employed to integrate mini-grids to the conventional grid thereby providing affordable access to electricity for communities that are hitherto unserved or underserved by the conventional grid. Some of the available smart grid technologies that have been identified to be suitable for integration purposes include advanced metering infrastructure (AMI), flexible alternating current transmission system (FACTS) and high voltage direct current (HVDC) transmission lines. A top-level schematic of how and where such technologies can be deployed is provided.

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%.

Research of independent DC electric field sensor with wireless power supply circuit

The DC electric field strength is an important electromagnetic environment parameter of the high-voltage direct current (HVDC) transmission line. Measurement results of the space electric field are dramatically affected by the size and the potential of the sensor. In addition, the battery-powered solution of electric field sensor (EFS) is not feasible, for the reason that in the high-voltage electric field environment, the replacement of the battery will bring security risks to the power system and workers. The wireless energy supply solution not only solves the problem of potential isolation of the sensor, but also ensures that the sensor can work continuously. Various wireless power transmission systems (WPTSs) were compared. Microwave wireless power transmission system (MWPTS) was selected as the power supply of EFS. The key boundary conditions of MWPTS were determined. Two microstrip receiving antennas were designed in this article. A good performance antenna with high permittivity substrate was designed, realising 4.26 dB gain with the area of 1 cm2 at 5.8 GHz. The antenna meets the energy requirements of the EFS which is 10 m far from the source.

Fault analysis method of integrated high voltage direct current transmission lines for onshore wind farm

Voltage source converter (VSC) based high voltage direct current (HVDC) transmission is most suited for the wind farm as it allows flexibility for reactive power control in multi-terminal transmission lines and transmits low power over smaller distance. In this work, a novel method has been proposed to detect the fault, identify the section of faults and classify the pole of the fault in DC transmission lines fed from onshore wind farm. In the proposed scheme, voltage signal from rectifier end terminal is extracted with sampling frequency of 1 kHz given as input to the detection, classification and section discrimination module. In this work, severe AC faults are also considered for section discrimination. Proposed method uses fuzzy inference system (FIS) to carry out all relaying task. The reach setting of the relay is 99.9% of the transmission line. Besides, the protection covers and discriminates the grounding fault with fault resistance up to 300 Ω. Considering the results of the proposed method it can be used effectively in real power network.

Characteristics of electrostatic discharges between metallic objects in the ion‐flow field under HVDC lines

Metallic objects under high-voltage direct current (HVDC) transmission lines will produce certain induced voltages due to the charging of ion flow. Electrostatic discharge (ESD) may occur between two objects at different potentials when they are close enough to spark under HVDC transmission lines. With a view to investigating such events in the ion-flow field generated by HVDC transmission lines, a laboratory study on the ESD characteristics of two metallic objects in a close approach to each other under HVDC line is conducted. Discharge current and induced voltage are monitored to analyse the effect of experimental parameters. The ground-level electric field without objects was measured, and the relationship between it and the voltage on objects was explored.

Interarea Oscillation Damping Control Using High-Voltage DC Transmission: A Survey

High-voltage direct current (HVDC) transmission lines are increasingly being installed in power systems around the world, and this trend is expected to continue with advancements in power electronics technology. These advancements are also bringing multiterminal direct current (MTDC) systems closer to practical application. In addition, the continued deployment of phasor measurement units makes dynamic information about a large power system readily available for highly controllable components, such as HVDC lines. All these trends have increased the appeal of modulating HVDC lines and MTDC systems to provide grid services in addition to bulk power transfers. This paper provides a literature survey of HVDC and MTDC damping controllers for interarea oscillations in large interconnected power systems. The literature shows a progression from theoretical research to practical applications. There are already practical implementations of HVDC modulation for lines in point-to-point configuration, although the modulation of MTDC systems is still in the research stage. As a conclusion, this paper identifies and summarizes open questions that remain to be tackled by researchers and engineers.