DC Polarity Reversal Research Articles

Simulation Analysis of DC Cable Terminal under Polarity Reversal Voltage

High voltage direct current transmission has the advantages of high transmission capacity, low loss, no simultaneous operation of the AC system on both sides, and small loss on the power grid in case of failure. The stability of synchronous operation does not limit its transmission capacity and distance, while avoiding the effects caused by low frequency oscillations. And, in the event of a blackout, it won’t cause massive power outages either, reducing losses. The DC transmission system may withstand both normal working DC high voltage and polarity reversal voltage, so it is extremely necessary to conduct theoretical analysis of the DC cable terminal under polarity reversal voltage. This article simulates the distribution of the electric field at the reverse polarity voltage of a DC cable terminal and analyzes its law of variation. The results show that when the conductivity of the main insulation to the reinforced insulation is equal to its dielectric permittivity ratio, the build-up of space charges can be effectively inhibited and guide relevant studies.

Study on electric field distribution of multi-layer dielectric under mixed voltage

The combination of field and circuit analysis method is used. Taking the multilayer flat dielectric structure as an example, the electric field distribution law of multilayer dielectric under mixing and polarity reversal voltage is studied by using the electromagnetic field theory. Then, according to the actual situation, two typical multilayer dielectric structures are selected, equivalent circuit model is established, and multilayer dielectric equivalent circuit experiment is carried out. At the same time, the equivalent circuit is simulated and analyzed by using the circuit simulation to further analyze the distribution law of electric field under various voltages. On this basis, the finite element simulation is used to simulate the double-layer plate dielectric structure model, which verifies the applicability of the finite element simulation method in the calculation of AC/DC hybrid electric field. Finally, the structural model of ±800kV converter outlet bushing is established, and the AC, DC, AC and DC superposition and polarity reversal voltage are loaded respectively. The electric field distribution in each medium of the complex insulation structure is calculated and analyzed accordingly. The results show that the finite element simulation method is simple and effective, and it is feasible to study the electric field distribution law and insulation structure optimization of the complex insulation structure under AC/DC mixed voltage. This paper studies E-field distribution law of multi-layer media under AC/DC mixed voltage, which has practical reference significance for the external insulation design of converter bushing, wall bushing and converter valve, and is of the great value for the actual DC power transmission and transformation project.

Effects of Oil Flow on Space Charge Behaviors in Oil–Paper Composite Insulation

Although the circulating flow of insulating oil can enhance heat dissipation in converter transformers, it can also influence the space charge distribution in oil–paper composite insulation. In this article, the oil–paper composite insulation between flat electrodes was used as the experimental model. Based on the pulsed electroacoustic (PEA) method, we investigated the effect of uncharged flowing oil on the space charge distribution in composite insulation under a dc electric field and polarity reversal. Under a dc electric field, as the oil flow velocity increased, the amount of interface charge between the oil and paper first increased and then decreased. During the polarity reversal process, the field distortion was aggravated at slower oil flows. These experimental results were explained in terms of the effect of oil flow on ionized charges in the oil.

Electric field control by permittivity functionally graded and superficially non‐linear conductivity materials for DC‐GIS spacer

This paper offers a simulation study on the application of a novel approach to control the electric field around a basin-type spacer by combining permittivity functionally graded materials (ε-FGM) and superficially non-linear conductivity materials (SNCM), namely multi-dimensional functional materials (MDFM). The permittivity distribution of the ε-FGM spacer is designed by the iteration optimization algorithm, and the epoxy/SiC composites with 30 and 60 phr doping contents are assigned as the coating materials of two SNCM spacers, namely SNCM30 and SNCM60. The MDFM spacer combines ε-FGM in its bulk and SNCM60 on its surface. The electric field regulation effect of each functional spacer is investigated under variable conditions, that is, DC steady state (DC-steady), DC turn-on state (DC-on), DC polarity reversal (DC-PR), and DC superimposed with switching impulse voltage (DC-SI). Results show that the applicability of ε-FGM and SNCM is limited to transient and stationary conditions, respectively. The MDFM spacer combines the advantages of ε-FGM and SNCM for adaptively relaxing the electric field under all the above conditions. It is hoped that this research will be beneficial to the development and application of spacers for the direct current gas-insulated switchgear (DC-GIS).

Iterative Finite Element Method Applied to the Nonlinear Electric Field of ±400kV Converter Transformer Barrier System

The DC steady-state and the polarity reversal (PR) are typical states of composite insulation in converter transformer barrier system, and the insulation medium has the strong E-field and temperature nonlinear characteristic, so the DC steady-state and polarity reversal E-field will be affected by the variation of electric parameters under temperature gradient. The FEM was applied to the quantitative E-field analysis considering electric and temperature nonlinear factors simultaneously. First, the nonlinear relationship between conductivity with E-field and temperature of medium has been obtained by experiments. Then, the iterative FEM was proposed and verified by coaxial insulation with analytical solution. Finally, the full-scale model of ±400kV converter transformer barrier system has been established, the DC steady-state and polarity reversal E-field has been calculated with the proposed method. The results show that under linear and nonlinear conditions the E-field is quite different, and E-field non-linearity can inhibit the local high electric spot, temperature non-linearity can reduce the E-field in the high temperature area and increase that in the lower temperature area. Under the temperature gradient condition, the local high E-field spot is easy to occur in the PR process. The iterative FEM and calculation results can provide references for the design of converter transformer barrier system.

Electric Field Simulation of Permittivity and Conductivity Graded Materials (ε/σ-FGM) for HVDC GIS Spacers

This paper discusses the application of the novel technology - functionally graded material (FGM), which combines both spatial permittivity (ε) and conductivity (σ) distributions with the aim to control the electric field around DC-GIS spacer. Some distribution types of ε and/or σ in the spacer bulk, such as U-type and graded to higher conductivity (GHC) are investigated through electric field simulation, in comparison to the uniform spacer with constant ε and σ distributions. The electric field relaxation effect of each distribution type of ε/σ-FGM under DC steady state, DC polarity reversal, DC-on, and lightning impulse voltage superimposed on DC steady state condition are obtained. The results show that ε/σ-FGM with U-type permittivity and GHC-type conductivity distribution of which low σ is applied near the high voltage side of the DC-GIS spacer is the most effective in reducing the maximum electric field under all stated conditions.

Inverse Power Law based Inclusive Life Model for DC Polarity Reversal Stresses

In this paper, carefully designed experiments are reported, which indicate that the change in frequency of voltage polarity reversals results in a change in slope of v-t characteristics on the inverse power law. The power law parameters are shown to be systematic functions of frequency of polarity reversals. Based on these results, the authors propose a comprehensive inverse power law for DC polarity reversal stress application, in place of conventional inverse power law. The proposed modification is examined with experimental data of authors apart from that available in literature and found to be working well. The model provides the maximum frequency of a reversal for maximizing the life of power equipment which may pave the way for deciding on the maximum frequency of voltage reversals.

Optimization of Lead Insulations for HVDC Converter Transformers

This paper explains design considerations for lead and lead-exit insulations of HVDC converter transformer for the purpose of automated design optimization. A HVDC converter transformer should at least deal with AC, DC, and DC polarity reversal stresses, which complicate designing process of insulation structures. Design considerations for those insulations are the types of electrical stresses and design parameters of insulation shapes. The design objective of the optimization is to ensure insulation robustness with reduced size of insulation structures. Optimization results showed 16.72% and 29.07% of length reduction with sufficient robustness, for lead and lead-exit insulations, respectively.

HVDC electrical insulation performance based on charge activity in oil-pressboard composite insulation structures

For more reliable HVDC power transmission systems, it is necessary to enhance HVDC electrical insulation performance in power equipment such as an oil-immersed AC/DC converter transformer. In this paper, the electric field distributions under AC, DC steady-state (DC-SS) and DC polarity reversal (DC-PR) conditions are discussed in oil-pressboard (PB) composite insulation structures. Firstly, based on discussion about the charge accumulation and charge dynamics at oil/PB interfaces, the contribution ratio of deposited charges under DC-SS to the following emerging electric stresses at DC-PR conditions are quantitatively clarified. Secondly, we found that the DC electric field distributions could significantly be controlled by PB arrangement in oil. In particular, both PB configurations and the conductivity ratios of oil and PB would play critical roles to determine electric field distribution, under DC-SS conditions. Furthermore, using an actual AC/DC converter transformer structure, electrical insulation performances under various testing conditions such as AC, DC-SS and DC-PR conditions were comparatively investigated. It was concluded that the most critical testing condition against breakdown strength could be the DC-PR conditions under the higher conductivity ratio of oil and PB.

HVDC electrical insulation performance in oil/pressboard composite insulation system based on Kerr electro-optic field measurement and electric field analysis

For the development of HVDC electrical insulation performance in liquid dielectric systems such as oil-pressboard (PB) composite systems in DC converter transformers, a Kerr electro-optic electric field measurement (Kerr-EOM) technique has been well established and will be considered as the key technique to be introduced. In this paper, the Kerr-EOM was experimentally applied to identify the electric field stress distribution and clarify charging activities in oil/PB composite insulation systems, under HVDC and DC polarity reversal (DC-PR) conditions. The physical mechanisms of charge movement and the amount of charge accumulation in liquid/solid insulation systems are also thoroughly investigated and discussed. Moreover, the breakdown discharge initiation by accumulated charges on the PB was observed and discussed together with the field measurement results. Secondly, by combining the Kerr-EOM with the electric field analysis using the Finite Element Method (FEM), the enhancement of the electrical insulation performance for HVDC applications was discussed and the critical testing voltage conditions in DC converter transformers were addressed. This paper is based on the Hans Tropper Memorial Lecture given at the IEEE DEIS, International Conference on Dielectric Liquids (ICDL) in 2017 in Manchester, UK.

Novel HVDC spacers by adaptively controlling surface charges – part ii: experiment

Charge accumulation on the surface of high voltage direct current (HVDC) spacers decreases their surface flashover voltage and affects the reliability of the insulation systems. In part I of this study, a novel HVDC spacer in which the surface charges can be adaptively controlled is proposed, and the aim of this paper is to experimentally verify the design. The experimental results of three kinds of model spacers, including a regular spacer, a spacer with a novel shape, and a spacer with charge adaptively controlled are provided. AC and DC surface flashover experiments and a polarity reversal test were performed. The effect of surface charges on triggering the surface flashover phenomenon are discussed. The results show that the surface flashover voltage increases both for the novel shaped spacer and the charge-adaptivelycontrolled spacer under application of AC bias, owing to a modification of the spacer shape that increases the creep distance. However, the surface breakdown voltage is the lowest for the novel shaped spacer under DC application and polarity reversal test. The charge adaptive control spacer displayed more advantages than the other samples under application of DC bias and in polarity reversal tests owing to its superior surface charge dissipation properties. It is hoped that this work could offer a new perspective for the design of a suitable spacer to avoid surface flashover when implementing HVDC energy transmission systems.

Design optimization for the insulation of HVDC converter transformers under composite electric stresses

This paper proposes a design optimization method for the insulation structure of a High Voltage Direct Current (HVDC) converter transformer, which is exposed to AC, DC, and DC polarity reversal (DCPR) stresses. Since DC and DCPR stresses provide significantly different electric field distributions, conventional transformer insulations designed for AC systems cannot guarantee safety against electrical breakdown. Focusing on main-winding and end-winding insulation including pressboard barriers and oil between windings, this paper presents a design solution based on genetic algorithm (GA). By exploiting modern computing power, the proposed approach can save a considerable amount of effort and time compared to conventional approaches which depend on experimental data and human experience. From the comparison with example reference insulation structures, we verified that the optimized structures maintain the required insulation performance with the reduction of length by 2.8% for the main-winding insulation and by 13.8% for the end-winding insulation.

Interface charge behavior of multi-layer oil–paper insulation under DC and polarity reversal voltages

Transformer oil, insulating paper, high voltage direct current (HVDC) bushings and on-load tap changer (OLTC) are essential parts of converter transformers. When the power flow of the HVDC system is reversed or a sudden outrage occurs, the charge field will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation damage. In this case, it is necessary to carry out the characterization of the oil–paper interface to avoid these consequences. This paper analyses the interface charge accumulation and dissipation characteristics of an oil–paper insulation system under DC and polarity reversal voltages. In this paper, induced electrification was used to charge the composite oil–paper insulation. The amplitude of DC and polarity reversal voltages applied in this paper was 4 kV, and paper with thicknesses of 0.08 and 0.13 mm was used. This research offers a theoretical exploration to optimize the transformer oil– paper insulation structure and prevent damage to its insulation systems. From the research on different layers and thicknesses of paper, it can be concluded that the decay is fast in the initial period and then becomes slower with the lapse of time until finally the decay curve tends to become flat. With an increase in the number of the paper layers, the interface charge density decreases. So in real life multilayer paper is used to avoid failures owing to its structural weakness. Regarding the DC polarity reversal effect, increasing the thickness of the paper is equivalent to adding paper layers. The dissipation rate of the positive interface charge is quicker than that of the negative one. In the polarity reversal test, the dissipation rate becomes smaller as the reversal time gets longer.

Insulation Characteristics of PPLP and Design of 250 kV Class HTS DC Cable

The development of high temperature superconducting (HTS) dc cable system has been promoted for the land long-term high-capacity power transmissions, submarine cables and renewable energies. Polypropylene laminated paper (PPLP) has been widely used as insulating material for HTS cable. To develop the PPLP-insulated dc cable, it is important to study the dc, dc polarity reversal and lightning impulse characteristics at a cryogenic temperature. In this paper, we discuss the electrical insulation characteristics of PPLP-insulated mini-model cable. We investigate the dc, impulse breakdown strength, dc polarity reversal, and dc voltage (V)-numbers of dc polarity reversal (N). Also, based on these results, the insulation design of 250 kV class HTS dc cable is performed.

Experimental and analytical study of the DC breakdown characteristics of polypropylene laminated paper with a butt gap condition considering the insulation design of superconducting cable

It has been reported that the insulation design under DC stress is considered as one of the critical factors in determining the performance of high-voltage direct current (HVDC) superconducting cable. Therefore, it is fundamentally necessary to investigate the DC breakdown characteristics of the composite insulation system consisting of liquid nitrogen (LN2)/polypropylene-laminated-paper (PPLP). In particular, the insulation characteristics under DC polarity reversal condition should be verified to understand the polarity effect of the DC voltage considering the unexpected incidents taking place at line-commutated-converters (LCC) under service at a DC power grid. In this study, to examine the variation of DC electric field strength, the step voltage and polarity reversal breakdown tests are performed under DC stress. Also, we investigate the electric field distributions in a butt gap of the LN2/PPLP condition considering the DC polarity reversal by using simulation software.

DC conduction and breakdown characteristics of Al2O3/cross-linked polyethylene nanocomposites for high voltage direct current transmission cable insulation

We have discussed a cross-linked polyethylene (XLPE) nanocomposite insulating material that is able to DC voltage applications. Nanocomposites, which are composed in polymer matrix mixed with nano-fillers, have received considerable attention because of their potential benefits as dielectrics. The nano-sized alumina oxide (Al2O3)/XLPE nanocomposite was prepared, and three kinds of test, such as DC breakdown, DC polarity reversal breakdown, and volume resistivity were performed. By the addition of nano-sized Al2O3 filler, both the DC breakdown strength and the volume resistivity of XLPE were increased. A little homogeneous space charge was observed in Al2O3/XLPE nanocomposite material in the vicinity of electrode through the polarity reversal breakdown test. From these results, it is thought that the addition of Al2O3 nano-filler is effective for the improvement of DC electrical insulating properties of XLPE.

The surface discharge and breakdown characteristics of HTS DC cable and stop joint box

A high temperature superconducting (HTS) DC cable system consists of a HTS cable and cable joint. The HTS DC cable should be electrically connected in joint boxes because of the unit length of HTS cable is limited to several-hundred meters. In particular, the stop joint box (SJB) must be developed for a compact cooling system. Polypropylene laminated paper (PPLP) and epoxy maybe used as insulating materials for HTS DC cable and SJB. To develop a HTS DC cable, it is necessary to develop the cryogenic insulation technology, materials and the joint methods. In this paper, we will mainly discuss on the DC and impulse characteristics of epoxy and PPLP in liquid nitrogen (LN2). The surface discharge characteristics of epoxy included fillers, PPLP and epoxy with PPLP composite (epoxy+PPLP) were measured under 0.4MPa. Also, the PPLP-insulated mini-model cable was fabricated and then DC, impulse and DC polarity reversal breakdown strength of mini-model cable under 0.4MPa were investigated.

고온초전도 직류 케이블용 절연재료인 PPLP의 직류 및 임펄스 절연 특성

To realize the high-Tc superconducting (HTS) DC cable system, it is important to study not only high current capacity and low loss of conductor but also optimum electrical insulation at cryogenic temperature. A model HTS DC cable system consists of a HTS conductor, semi-conductor, cooling system and insulating materials. Polypropylene laminated paper (PPLP) has been widely adopted as insulating material for HTS machines. However, the fundamental insulation characteristics of PPLP for the development of HTS DC cable have not been revealed satisfactorily until now. In this paper, we will discuss mainly on the breakdown characteristics of 3 sheets PPLP in liquid nitrogen (<TEX>$LN_2$</TEX>). The characteristics of the diameter, location of butt-gap, distance between butt-gap length, pressure effect, polarity effect under DC and impulse voltage were studied. Also, the DC polarity reversal breakdown voltage of mini-model cable was measured in <TEX>$LN_2$</TEX> under 0.4 MPa.

Evaluation of DC electric field distribution of PPLP specimen based on the measurement of electrical conductivity in LN2

High temperature superconducting (HTS) cable has been paid much attention due to its high efficiency and high current transportation capability, and it is also regarded as eco-friendly power cable for the next generation. Especially for DC HTS cable, it has more sustainable and stable properties compared to AC HTS cable due to the absence of AC loss in DC HTS cable. Recently, DC HTS cable has been investigated competitively all over the world, and one of the key components of DC HTS cable to be developed is a cable joint box considering HVDC environment. In order to achieve the optimum insulation design of the joint box, analysis of DC electric field distribution of the joint box is a fundamental process to develop DC HTS cable. Generally, AC electric field distribution depends on relative permittivity of dielectric materials but in case of DC, electrical conductivity of dielectric material is a dominant factor which determines electric field distribution. In this study, in order to evaluate DC electric field characteristics of the joint box for DC HTS cable, polypropylene laminated paper (PPLP) specimen has been prepared and its DC electric field distribution was analyzed based on the measurement of electrical conductivity of PPLP in liquid nitrogen (LN2). Electrical conductivity of PPLP in LN2has not been reported yet but it should be measured for DC electric field analysis. The experimental works for measuring electrical conductivity of PPLP in LN2 were presented in this paper. Based on the experimental works, DC electric field distribution of PPLP specimen was fully analyzed considering the steady state and the transient state of DC. Consequently, it was possible to determine the electric field distribution characteristics considering different DC applying stages including DC switching on, DC switching off and polarity reversal conditions.

油浸絶縁の直流極性反転時の過渡電界と絶縁強度の検討

油浸絶縁の直流極性反転時の過渡電界と絶縁強度の検討