HVDC Research Articles

Effect of Li1.3Al0.3Ti1.7(PO4)3 on semiconductive composites: Positive temperature coefficient (PTC) performance and inhibition of space charge injection

The semiconductive layer located between the core and insulation layer of a high-voltage DC (HVDC) cable can inhibit the accumulation of space charge in the insulation layer. This helps increasing the electric field inside the cable, thus improving the stress distribution of the electric field on the inner and outer surfaces of the insulation layer and the electrical strength of the cable. An inevitable positive temperature coefficient effect (PTC) in the semiconductor layer leads to aging failure of the insulation layer. To improve this phenomenon, the semiconductive shielding layer was modified for suppressing its PTC effect and reducing the space charge injection into the insulation layer. Li1.3Al0.3Ti1.7(PO4)3(LATP) was prepared via a solid-phase reaction and added to a composite consisting of carbon black (CB), ethylene vinyl acetate copolymer (EVA), and low-density polyethylene (LDPE). Resistivity, thermally stimulated depolarization current (TSDC), and pulsed electroacoustic (PEA) measurements were performed on the modified sample. The results showed that when the LATP content was 4 wt%, the modification effect of the semiconductive shielding layer was most effective, the PTC effect was reduced by 11%, and the space charge injection into the insulating layer was improved significantly.

Comprehensive layout optimization of the transmission system in a deepwater wind farm cluster

Compared to nearshore wind farms, deepwater offshore wind farms have better wind resources and broader development space and will be the main trend of wind power development in the future. Due to the long distance from the shore, the investment and operation costs of the transmission system in a deepwater wind farm cluster are high and the electricity generated by the cluster is commonly transmitted by high-voltage direct current transmission technology to decrease power losses. For a given converter station, the comprehensive optimization of the layouts of the substations, AC export cables from the substation to the converter station, and cable arrays within each wind farm are complex and have not been well studied. This paper proposes a multi-level nested optimization strategy to minimize the total cost of the collection cables for a deepwater offshore wind farm cluster. The proposed method applies the Informed-RRT* method to optimize the layout of AC export cables while the cable arrays are arranged by Prim’s algorithm. During the optimization process, the gradient descent algorithm is used to optimize the positions of the substations, obtaining the transmission system scheme with the minimum cable investment. For a certain deepwater wind farm cluster, the total cable cost after optimization is reduced by 3.58%, which indicates the effectiveness of this method.

Power System Analysis of Moving from HVAC to HVDC in the Presence of Renewable Energy Resources

As global energy consumption continues to increase, increased utilization and adaptation of renewable energy resources have tremendously increased over the last decades. Unfortunately, despite the many benefits of renewable energy resources, the intermittent nature of generation and the far distance of large installations from demand centers have tremendous effects on the connecting grid’s stability. In this study, high-voltage direct current (HVDC) systems are proposed as a solution for stable and reliable grid operation in the presence of large renewable energy installations. This research investigates the deployment of an HVDC system into an entire network rather than studying it as an isolated radial system. Various power system analysis functions for both static and dynamic conditions are used to study the effect of integrating an HVDC system on the overall network’s stability. To verify the proposed approach, Jordan’s national electric grid was modeled and used as a case study. The results show when deploying HVDC transmission, losses are reduced by 70% from the baseline case, in addition to better handling of contingency events and enhanced grid’s stability when examining the generator’s rotor angle and speed. Rigorous modeling and simulations of the proposed system structure show the feasibility and prove the advantages of modern HVDC systems over HVAC counterparts.

A Measurement Method for the Charging Potential of Conductors in the Vicinity of HVDC Overhead Lines Based on a Non-Contact Electrometer

Charging potential appears when a conductor insulated to the ground is exposed to an electrostatic field. A modified measurement method based on a non-contact electrometer is presented in this article. This system is appropriate for measuring the charging potential of a conductor near high-voltage direct-current (HVDC) overhead lines. Compared with the contact measurement method, the modified method is hardly affected by the internal resistance of the electrometer, which helps ensure measurement accuracy in the electrostatic field. In contrast with the traditional non-contact electrometer, the electric field generated by HVDC lines and the space charges around the electrometer probe are shielded using a grounding cage. The effectiveness of the modified measurement method was verified via experiments. The impacts of the structure and position of the measurement system on the measured charging potential are discussed.

Surge arrester allocation for lightning protection of VSC based HVDC prototype

High-voltage direct current (HVDC) systems, based on voltage source converters (VSC), enable asynchronous interconnection of grids with different frequencies and contribute to grid stability by controlling power quickly and precisely. Effective protection of these expensive systems, which must be in continuous operation, increases their lifetime. An overvoltage protection study is needed to ensure the proper protection of HVDC systems that contain multiple devices with varying insulation levels. In this study, lightning protection studies on a prototype VSC-based HVDC (VSC-HVDC) system are explained. In line with this, lightning impulses were modeled and applied to various points in the PSCAD model of the prototype VSC-HVDC, and the response of the system combined with the protection equipment was examined. All stages of protective equipment selection were explained step by step. The suitability of the selected protective equipment was verified based on the simulation results. After running dozens of scenarios, the protection coordination was optimized, and the critical points were determined instead of installing surge arresters at each connection point. In this way, a significant cost benefit was obtained for the investment in protective equipment. Finally, the results of the lightning impulse tests performed to verify the insulation of the VSC-HVDC prototype are reported.

Multi‐objective optimization method for control parameters of flexible direct current transmission converters based on intelligent algorithms

AbstractIn order to meet the current needs of power transmission, people have turned their attention to direct current (DC) transmission. When the alternating current(AC) power grid (PG) fails, it is easy to cause commutation failure in the DC system, thereby increasing the fault scope of the system. Therefore, this article adopts a flexible DC transmission system. Flexible DC converters have many control parameters, and there are mutual influences and conflicts between these parameters. By applying intelligent algorithms to multi‐objective optimization (MOP), these complex MOP problems can be effectively solved and an optimal set of control parameter combinations can be found. This method has an important contribution and novelty for optimizing the converter control parameters of flexible DC transmission systems. This article achieves fast and accurate MOP of the control parameters of flexible DC transmission converters by continuously detecting the similarities between antibodies and maintaining the diversity of the population. The research results show that the average DC side current is about 405A in the first 20 seconds, and the DC side current fluctuates regularly after 20 seconds. This article lays the foundation for establishing a real‐time simulation model of flexible DC transmission systems suitable for real‐time simulation systems.

A parameter optimization method of multi-layer nested slit resonator for the converter transformer noise in high voltage direct current converter station

We propose a parameter optimization method of multi-layer nested slit resonator (MNSR) for the converter transformer noise in a high voltage direct current (HVDC) converter station. With the optimization objective of achieving quasi-perfect sound absorption at the target frequencies and the structural parameters of MNSR as the optimization variables, the optimization model of the structural parameters of MNSR is established and then solved by the sequential quadratic programming algorithm (SQP). The theoretical and simulation results of the four validation cases show that the optimized MNSR can achieve quasi-perfect sound absorption at all target frequencies, which is consistent with the optimization objective. The parameter optimization design method provides an efficient method to design a dual-frequency quasi-perfect sound absorber for the converter transformer noise.

LoRaWAN-based IoT protocol for three levels central protection scheme in MT-HVDC networks with ANFIS-PSO restoration algorithm

A new three-level adaptive central protection scheme has been introduced for multi-terminal high voltage direct current (MT-HVDC) transmission grids. The primary protection is based on a differential relay, while two backup protection levels are proposed: a local dv/dt under-voltage relay and a centralized overcurrent relay at the AC grid side. The coordination of these three protection levels is managed based on a convenient coordination time. The communication between the central protection unit and the Fault Isolation Devices (FIDs) is established using the LoRaWAN-based Internet of Things (IoT) protocol. In case of fault clearance triggered by the third protection level, the faulty line is detected and isolated using the adaptive neuro-fuzzy inference system (ANFIS) algorithm. The restoration process of the healthy lines focuses on minimizing load shedding and line power loss. The protection scheme proposed in this article has been implemented and tested under various fault conditions using the MATLAB/Simulink® environment. The results demonstrate the effectiveness and efficiency of the proposed protection scheme in detecting and isolating faulty lines in MT-HVDC transmission grids, along with a rapid restoration technique.

Inverter design trade-off for photovoltaic power generation

Photovoltaic (PV) power generation is a very important way of energy conversion. It can convert solar energy into electricity. As the core photovoltaic power generation device, the microinverter design has many challenges. Limitations that need to be considered include high-frequency leakage current generated by parasitic capacitance, the front DC voltage gain of the inverter, and switch components’ number and power consumption. In many cases, these limitations are difficult to solve perfectly simultaneously in a standard H-bridge or three-phase inverters. A front boost converter requires a high DC voltage gain with fewer switches, and Z-source or split-source structures can be applied. But these structures can lead to high leakage current. A traditional half-bridge inverter can solve the problem but with low efficiency. H5 or HERIC full-bridge inverter for PV can eliminate leakage current while more switches are used. A dual-boost common-grounded H-bridge topology can remove the leakage current and achieve a high DC voltage gain with only four switches. However, the power consumption of different switches varies greatly, which may cause an imbalance in heating and damage the device.

A hybrid DC-DC modular multilevel converter with capacitors parallel connectivity for arm energy balancing

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms’ capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach.

Design of Optoelectronic Transformer for Its Application in Traveling Wave Protection of Power Grid

As an important foundational link of the power system, the reliable measurement and accurate data of the metering module are the important foundation and support for energy metering. As a new type of optoelectronic current transformer, magneto-optical crystal-based optoelectronic current transformer effectively avoids the defects of traditional electromagnetic current transformers and becomes an ideal equipment for measurement in power systems. This paper proposes an improvement method to enhance the temperature stability of optoelectronic current transformers, using a magneto-optical crystal sensor head based on Faraday effect to limit the fiber length to within 140 m. It considers the influence of a uniform temperature field on magneto-optical crystal materials to process data reasonably to ensure normal incidence of the incident beam. The magneto-optical crystal material in the non-uniform temperature field has non-uniform deformation due to thermal expansion and cold contraction. Here, a Karamay No. 45 cooling oil is used for filling and sealing. In the experiment, the temperature stability of the current transformer is tested. Firstly, the wiring method is designed. Then, the corresponding test system is designed. After improvement, there is no drift phenomenon in the ratio difference and angle difference of all points of the photoelectric current transformer within the testing temperature range, and the error difference is stable. The principle of identifying faults within the area is elaborated, and a directional longitudinal traveling wave protection method for direct-current transmission lines is designed based on the differential output characteristics of the designed photoelectric transformer. Through experiments, the transformer in this design can effectively identify potential traveling wave fault types and fault phases.

Static electric field (SEF) exposure promotes the proliferation of B lymphocytes

With the rapid development of ultra-high voltage direct current (UHV DC) transmission technology, the intensity of electric fields in the surrounding environment of UHV DC transmission lines significantly increased, which raised public concerns about the potential health effects of electric fields. Previous studies have shown that the exposure of electromagnetic field was associated with cancer. B lymphocytes can produce autoantibodies and tumor growth factors through proliferation, which contributes to the development of cancer. Therefore, this study explored the effect and mechanism of static electric field (SEF) generated by DC transmission lines on the proliferation levels of B lymphocytes. Male mice were exposed to SEF. After the exposure of 7 and 14 days, the proliferation levels of B lymphocytes in the spleens of mice were measured, respectively. To validate biological effect discovered in animal experiments and elucidate the mechanism of the effect from the perspective of signaling pathways, lymphocytes were exposed to SEF. After the exposure of 24, 48 or 72 h, the proliferation levels of B lymphocytes, the expression levels of key proteins and cell cycle were determined. This study found that SEF exposure activated NF-κB pathway by stimulating ERK1/2 pathway and promoted B lymphocytes to enter S phase from G0/G1 phase. Meanwhile, SEF exposure also promoted B lymphocytes to enter G2 phase. Namely, SEF exposure significantly promoted the proliferation of B lymphocytes. This discovery provided theoretical and practical support for the prevention or application of negative or positive effects caused by SEF exposure and provided directions for future research.

A Dynamic Nonlinear VDCOL Control Strategy Based on the Taylor Expansion of DC Voltages for Suppressing the Subsequent Commutation Failure in HVDC Transmission

Subsequent commutation failure in high-voltage DC transmission systems seriously emphasizes the safe and stable operation of power systems. Via analyzing the mechanism of commutation failure and the principle of voltage-dependent current order limiter (VDCOL), this paper proposes a dynamic nonlinear VDCOL control strategy based on the Taylor expansion of DC voltage for suppressing subsequent commutation failure. To solve the problem of fluctuating DC current command value caused by a large drop in DC voltage, this paper constructs a nonlinear VDCOL control that can be dynamically adjusted according to the AC bus voltage level, and Taylor expansion of DC voltage is used to obtain its first-order and second-order differential components. Different scales of differential elements are chosen to predict the DC voltage compensation value while balancing sensitivity and accuracy. The compensated DC voltage, used as the starting voltage of VDCOL, is input to the VDCOL control constructed in this paper to suppress the subsequent commutation failure of the transmission system by reducing the fluctuation of the current command value. Finally, the standard test model of HVDC is established based on the actual parameters, and the simulation results show that the Method proposed in this paper has an effective suppressing effect in the case of single-phase or three-phase faults of different severity and is conducive to the restoration of the system power transmission.

Adjustments for an HVDC link fault detector: practical considerations

In this work, practical considerations regarding the use of a fault diagnosis scheme based on a distributed adaptive estimator for the high-voltage direct current (HVDC) transmission line within a simplified model of the DC link are presented. Due to transients and steady-state ripple caused by unmodeled nonlinearities in the HVDC system's conversion part, parameter adjustments of the distributed estimator, including gains and thresholds, are performed to successfully determine the fault magnitude and position within the line through simulation studies. To validate the proposed scheme, numerical simulations of the diagnostic scheme are compared with two HVDC links: a simplified one and the CIGRE (International Council on Large Electric Systems).

Optimal transmission switching strategy for multi‐infeed DC receiving‐end power network to prevent over‐limit of short‐circuit current

AbstractA large‐scale AC/DC hybrid power network has recently been constructed in China because of the fast development of high voltage direct current (HVDC) technology. The receiving AC system must have sufficient system strength to guarantee the safe operation of the DC side. The paradox is that raising the AC side system's strength will increase the short‐circuit current (SCC) level, endangering the safety of the system. In order to resolve the contradiction between the system strength of multi‐infeed DC receiving‐end power networks and SCC, this paper proposes an optimal transmission switching strategy. Firstly, the system strength is characterised by the multi‐infeed short‐circuit ratio (MISCR), followed by constructing a comprehensive constraint set that considers the MISCR and SCC simultaneously. Secondly, to optimize SCC with MISCR and N−1 safety constraints, a mixed integer linear programming model is developed. Finally, to enhance the computing performance for large power systems, a new connectivity constraint set and decision space reduction techniques are provided. The proposed method is tested for effectiveness using the IEEE 30‐bus and IEEE 118‐bus systems.

High gain coupled inductor SEPIC based boost inverter using extended SPWM

This research work designs a high gain coupled inductor SEPIC (CI-SEPIC) based boost inverter. This topology presents low switching voltage stress, high output DC and AC voltage and highly efficient system, which makes it suitable for renewable energy applications. A seventh order system is obtained through dynamic modelling of the CI-SEPIC converter for which PID controller is designed to track the reference value. The extended sine-wave pulse width modulation (ESPWM) has maintained the high gain without compromising the efficiency and ensured low voltage stress on the switching devices. Simulation and experimental results validated that CI-SEPIC based boost inverter with ESPWM achieved high dc gain of 15.98, boost factor of 25, exceptional dc-ac coupling with minimum possible capacitance of 3.3uF, decreased device stress by 33% and smaller size of the complete system.

Study on the Formation and Controllable Movement of Polymer Liquid Columns Based on Dynamic Control of Spatial Electric Field Distribution.

Liquid deformation and motion are very common natural phenomena and of great value in various practical applications. In this study, a dielectric fluid column formation and directional flow phenomenon are presented. Dielectric fluid can grow upward to form a liquid column through a spatial electric field and realize directional and controllable operation of the liquid column by regulating spatial electric field distribution. First, the adjustable electric field space is constructed by connecting the two parallel electrodes to the high-voltage DC power supply. Then, the regional electric field distribution was adjusted by the upper plate graphic and power supply regulation to drive the polymer liquid on the lower plate electrode to form a liquid column at different positions. The results show that the polymer liquid column can be driven by the spatial electric field distributed dynamic control method and that the height and the narrowest width of the liquid column are directly controlled by the voltage. With the experiment conditions that the distance between two parallel electrodes is 5-15 mm, the formation of liquid columns with a height of 5-15 mm can be controlled. In addition, the liquid column can be driven by adjusting the on-states of different conductive regions. When the voltage is 10 kV, the liquid column directional movement speed can reach 1 mm/s. The higher the voltage, the faster the directional movement. The research results can be used as producing polydimethylsiloxane stamp, localized heating and temperature control, fabricating a pulsating heat pipe, and so on.

The enhanced physical properties of XLPE insulation for high voltage cable based on LDPE/LLDPE or LDPE/HDPE blends

Exploring methods of improving the performance of high-voltage cable XLPE insulation is a hot issue in high-voltage and insulation technology. This paper presented a new method to improve the physical properties of XLPE by doping low-content polyolefin materials, such as LLDPE or HDPE. The new XLPE insulation has significantly improved high-temperature mechanical properties and slightly improved room-temperature mechanical properties. The optimal thermal elongation properties were achieved with a 1 phr content of HDPE and LLDPE, while DC conductivity, DC breakdown strength, and dielectric properties were stable or slightly improved. The properties mentioned above showed a decreasing trend at higher content. This study demonstrated that adding HDPE or LLDPE to LDPE increases the physical entanglement of XLPE molecular chains, which has a similar effect to crosslinks formed by chemical crosslinking in enhancing the mechanical properties of XLPE insulation. This suggests that there is a potential to reduce the concentration of crosslinking agents during XLPE cable manufacturing while maintaining electrical properties and improving mechanical properties at elevated temperatures. It provides a new idea for improving the properties of XLPE insulation for high-voltage DC cables.

Optimization of Switching Control and Microgrid Energy Management System with Alternate Arm Converter Based on Bacterial Foraging Algorithm

The global demand of electrical power has increased enormously due to various reasons such as fast-changing and challenging technologies, climatic change, economic growth, and lifestyle of mankind. Due to global warming issues and the depletion of conventional sources of power generation, the utilization of renewable energy sources has drastically increased. So, many challenges exist in integrating the microgrid with AC grid and load. The alternative arm converter (AAC) is among the most innovative converter topologies used in high voltage direct current (HVDC) applications. This research work presents a new control strategy to generate pulses to trigger the switches in the AAC in a proper sequence to obtain a smooth output waveform. The AAC output is controlled by implementing various controllers such as the proportional-integral-derivative (PID) controller, fractional order PID (FOPID) controller, and FOPID controller tuned by metaheuristic algorithm-bacterial foraging optimization technique (BFOT). Also, a comparative analysis is performed based on the spectral analysis of the output voltage obtained. In comparison to other controllers, the FOPID controller optimized by the bacterial foraging optimization technique (BFOT) produced the least total harmonic distortion (THD) of the AAC output voltage. In addition, this paper also discusses about the performance and analysis on the design of an energy management system (EMS) to optimally utilize the energy sources such as PV system, wind system, and battery based on their availability which feed the AAC. The energy management system controls the entire integrated system in association with an integrated CUK-SEPIC converter to fulfil the load demand at the point of common coupling, either from the microgrid or the AC grid. A nine-level AAC model is designed integrating the microgrid, grid, and industrial loads with an energy management system using MATLAB/Simulink. The performance parameters of the entire model are analysed at every stage to provide stabilized output to meet the load demand.

Adaptive power capability control scheme for voltage source converter to improve transient stability

Applications of Power Electronic Devices (PED) are rapidly increasing because of their controllability. A Direct Current (DC) transmission system is used for the interconnection of Renewable Energy Sources (RES), and a DC grid research reached the stage of demonstration. Since power electronic devices are controllable, as opposed to conventional ac power facilities, various studies have been carried out on control schemes. Based on fast response speed, the device can affect the stability of the power system by improving the transient stability according to the control method. However, the operation range of converters is limited based on the normal state of the AC system. Therefore, the entire converter capacity cannot be used, and the same standard is applied even in the transient state. In this paper, a novel control scheme is proposed in order to improve the voltage and frequency stability by using the maximum capacity of the converter. The control scheme calculates an Adaptive Power Capability Index (APCI) based on the range of variation of frequency and voltage in the transient state, then applies this index to the controller. In order to verify the new control scheme, a dynamic model is developed and verified in a Transient Stability Analysis (TSA) program. Following the verification of the model, the model is applied to the actual power system data and the power system stability is analyzed.