High Voltage Side Research Articles

Research on Non-destructive Testing Method for Aging of Sheds of Composite Insulators

As an important part of external insulation of transmission and distribution lines and substation equipment, silicone rubber insulators are generally used because of their strong anti-pollution flashover ability, excellent hydrophobicity, and lightweight. It is one of the effective means to obtain the aging degree of sheds quickly and accurately to prevent operation accidents. From the micro and macro perspectives, firstly, this paper uses the traditional scanning electron microscopy and dielectric loss factor test to classify the aging degree of 11 high-voltage side sheds samples of silicone rubber insulators running on-site for 0~11 years. According to the different experimental principles, the aging degree of each experimental method is divided into four grades. Then, the detection method based on terahertz time-domain spectroscopy is proposed to realize a nondestructive evaluation of the aging grades of sheds of silicone rubber insulators. Finally, through comparison, it can be found that the THz absorption coefficient spectra can find the characteristic quantity of classification of aging levels of silicone rubber materials: The aging levels of the silicone rubber samples increase, and the absorption peak at 1.32 THz in the absorption spectra show a downward trend. According to the characteristic quantity, 11 samples can also be divided into four aging grades, and the results are consistent with the previous experiments.

DC Impedance Modeling of Push-Pull DC Auto-Transformer for MMC and LCC HVDC Interconnections

The push-pull DC autotransformer (PPDAT) is a potential power equipment to interconnect the MMC and LCC HVDC systems. However, the harmonic stability of the PPDAT remains an unsolved challenge. In this paper, considering the dual closed-loop control and bidirectional DC voltage operation of the PPDAT, its DC impedance models are proposed using the harmonic transfer function (HTF) method. Both the DC impedance models at the high/low voltage side considering the harmonic coupling and internal dynamics of the hybrid MMC/LCC HVDC system are deduced. The accuracy of the detailed DC impedance models is validated in PSCAD/EMTDC simulations. The impedance analysis under bidirectional DC voltage operation scenarios reveals that the DC voltage polarity affects the low frequency region. Besides, the main impact of PPDAT's parameters on the wideband impedance characteristics is disclosed. It shows that unreasonable parameter design will bring negative damping characteristics near the resonance point (RP). Moreover, stability analysis considering the interaction of PPDAT and LCC-HVDC at the high-voltage side is conducted. The results indicate that the potential resonances can be readily predicted through the Nyquist diagrams and the negative damping of the PPDAT may cause instability at RP.

An Integrated Power Decoupling Method for Single-Phase EV Onboard Charger in V2G Application

Instead of bulky passive filters, the active power decoupling (APD) method can be adopted to suppress the 2nd-order ripple power in the DC-bus of the single-phase electric vehicle (EV) onboard charging system. However, the traditional APD methods require additional power switches and energy storage devices, which increases the cost and significantly reduces the power conversion efficiency. To tackle these problems, an integrated method of utilizing the auxiliary power module to form a series-connected APD circuit is proposed in this paper. Without additional switches and energy storage devices, the APD circuit only needs to compensate less than 10% of the rated power, while realizing soft-switching operation. In addition, the corresponding control method can further reduce the voltage stress of the switches on the high-voltage side of the converter, and increase the efficiency of the system. Finally, a 2kW single-phase charging system prototype is built to verify the feasibility and effectiveness of the proposed method.

A Phase-Shift-Modulated Resonant Two-Switch Boosting Switched-Capacitor Converter and Its Modulation Map

In this paper, a Phase-shift-modulated (PS) Resonant Two-switch Boosting Switched-capacitor Converter (RTBSC) and its modulation map are proposed. Compared with the traditional RTBSC, all diodes are replaced by active switches and then phase-shift modulation is applied. The advantages of the modified topology are obtained as follow. 1) The voltage gain can be regulated efficiently either below or above the nominal value even at light-load condition, making it suitable for applications with voltage fluctuations; 2) The conduction loss is reduced by replacing all diodes in RTBSC with low turn-on resistance transistors, especially for high-order and high-gain configuration; 3) All switches achieve the zero-voltage-switching (ZVS) turn-on operation; 4) The soft-charging property smooths the current profile. Aside from the topology modification, a comprehensive analysis of operation principle, voltage gain, component stress and ZVS region is given. On the basis, a modulation map with the optimal operation region is proposed. It provides a guideline to operate and design such converters in the target gain range, low component stress and ZVS region, making up for the lack of modulation map and design guideline in aforementioned literatures. A 120W prototype with low-voltage side 43-60V and high-voltage side 150V was designed to verify the above analyses.

Study on the Simulation of the EMU Body Operating Main Breaker Overvoltage and High-Voltage Side Suppression Method Based on the 3D Structure

Current circuit simulation research on EMU (Electric Multiple Unit) overvoltage has focused on overvoltage amplitude distribution and the oscillation frequency of the operating main circuit breaker overvoltage. However, when studying the suppression method, the overvoltage amplitude and frequency characteristics must be considered simultaneously, and few studies have reported on the overvoltage frequency distribution. In this paper, a new simulation model of train-operational main breaker overvoltage based on the 3D structure of the EMU body was proposed, which provided guidance for the study of overvoltage suppression in operational main circuit breakers. This model simulated the transient overvoltage characteristics and distribution of the vehicle body when the train operated the main circuit breaker. The simulated results showed that the first vehicle had the highest operating overvoltage amplitude, and the fourth vehicle had the lowest. The overvoltage amplitudes of vehicles 1–4 were all above 1 kV, and the main overvoltage oscillation frequencies of each vehicle were at 0.70 MHz and 1.30 MHz. The accuracy of the distribution characteristics and spectral characteristics of the operating main breaker overvoltage was verified by static tests on trains, and the simulation was considered to be in agreement with the test. Aiming at the problem that there was no universal suppression method for the operating main breaker overvoltage, a general suppression method for the overvoltage on the high-voltage side of the EMU was studied, and an overvoltage suppression device based on a grounding inductor was proposed. The inductance parameter requirements of the suppressor were theoretically analyzed, and the validity of the design of the inductance suppressor was experimentally verified.

Study on fault waveform library of PT high voltage side fuse in hydropower station

In the arc-suppression coil grounding system of a medium voltage distribution network, high voltage side voltage transformer insurance fusing faults frequently occur, including fusing during startup and grid connection, fusing during operation, fusing during the shutdown, etc., which will directly affect the accuracy of measurement, metering, protection, and other secondary equipment. In this paper, we used the calculation of electromagnetic transient EMTP-ATP software, theoretical analysis of the typical hydropower station wiring and parameters of the medium voltage system, set up the improved model of neutral point grounding system, simulation of single-phase earth fault line and disappeared in the transient process, calculation of PT transient current, to verify that the way to study the mechanism of frequent fusing of fuse in the PT. Typical fault waveform databases are established under different bad working conditions (ferromagnetic resonance, low-frequency nonlinear oscillation, lightning wave invasion, etc.).

Online Power Transfer Regulation Between Transmission and Active Distribution Systems for High-Voltage Support

Regulation of power transfer between High-voltage Side (HVS) and Distribution Power Systems (DPS) is an urgent issue for modern power grids. During light loading, active DPS can export the extra power to the transmission system and may create voltage rise at HVS. In other words, active DPS are highly required to provide ancillary services to HVS during its unexpected failures. Therefore, this study suggests an online centralized control approach to satisfy the international Demand Connection Codes (DCC) at Transmission/Distribution interconnection point or fulfill the power requests from HVS. A linear power-flow taking into account the voltage-dependence characteristics of electric loads is used for problem formulation. The problem is formulated as a mixed integer quadratic optimization problem (MIQP). The MIQP problem can easily be solved to its optimality solution while reducing the complex computations. The proposed method can regulate both active and reactive power transfer at the interconnection point and thus it can be operated at different modes. The proposed control method uses the Distributed Generation (DGs), Capacitor Banks (CBs), and controllable loads as control variables. The use of controllable loads can increase the responsibility level of DPS in satisfying the DCC requirements. The suggested approach was checked on a 11 kV 77-bus active DPS.

Battery-Friendly DC–DC Converter With Ripple-Free Current Modulation Strategy

In this article, a battery-friendly dc–dc converter with a ripple-free current technique is proposed interfaced battery energy storage system (BESS). Two-phase interleaved circuits are adopted to cancel the current ripple on the BESS side. On the high voltage (HV) side, the circuit combined with a half-bridge circuit and full-bridge circuit is employed to achieve voltage-second balance control. With the associated control strategy, the power control variables can be decoupled. The voltage-second balance control on two ends of the leakage inductor is realized by the adjustment of the duty cycle. After that, the transferred power only changes monotonously along with the change of phase shift angle. The zero-voltage switching (ZVS) conditions are analyzed in detail to ensure that the ZVS of all switches can be achieved through parameter design. Moreover, a comparison is conducted to illustrate the merits of the proposed converter. The theoretical analysis has been fully validated experimentally by an experimental prototype of 40–60 V to 400 V/1 kW operating at 50 kHz. The accordance between the theoretical analysis and experimental verification further testifies to the effectiveness.

DESIGN AND ANALYSIS OF BIDIRECTIONAL BUCK-BOOST CONVERTER FOR GRID TO PHEVAND PHEV TO HOME

Nowadays, the use of Electric Vehicles (EV) is rapidly increasing. The batteries of the EV are charged when its demand is low, and the energy would then be stored to act as a kind of reserve that could be used when necessary. A new design of “bidirectional DC-DC buck boost converter” is suggested for transferring electrical power between the grid and an electric vehicle, as well as between an electric vehicle and the residence. While using grid energy to charge the car battery, the Grid to Vehicle (G2V) mode utilizes a buck mode DC-DC converter operation. The Vehicle to Home (V2H) function uses the battery's stored energy to power dwellings off the grid when the DC-DC converter is running in boost mode. The battery's Status of Charge is determined using an enhanced Kalman filter. Proportional-integral controllers are used in bidirectional buck boost converters to maintain a consistent voltage on both the high voltage and low voltage sides. Phase angle control is used for G2V and V2H operations on the H-Bridge inverter that the DC-DC converter is coupled with. The proposed circuit is simulated in MATLAB, and the results are reviewed.

Simulation and Protection of Reignition Overvoltage in Wind Farm Considering Microscopic Dielectric Recovery Process of Vacuum Circuit Breaker

The high amplitude and steep overvoltage generated by the breaking of the vacuum circuit breaker in the wind farm damages the inter-turn insulation of the transformer. There is a certain difference between the simulation results of the traditional reignition model and the measured overvoltage. It is necessary to improve the simulation model to simulate the overvoltage condition of the transformer more accurately and then select appropriate overvoltage protection measures. In this paper, based on the physical process of dielectric recovery during the opening process of the vacuum circuit breaker, a model of dielectric strength recovery is built to simulate the arc reignition of the vacuum circuit breaker. The model was applied to compare the overvoltage protection effects of RC snubbers, surge arresters, and choke coils. The simulation results show that the overvoltage amplitude and reignition times calculated by the model proposed in this paper are closer to the measured values. Compared with the traditional linear curve reignition model, the accuracy was increased by 24% and 51.2%, respectively. The parameter value of RC snubbers, the connection mode of surge arresters, and the combination mode of choke coil have an influence on overvoltage suppression. Finally, a suitable suppression scheme is proposed by installing a combined arrester on the high-voltage side of the transformer and connecting a choke coil in series, which can limit the phase-to-ground voltage and the phase-to-phase voltage to 2.43 p.u and 3.24 p.u, respectively, and reduce the steepness from 157.2 kV/μs to 22.3 kV/μs.

A DAB-Based DC Transformer for Multiterminal HVdc Dynamic Simulation Platform With Short Circuit Tolerance

The dc transformer (DCT) can be used to achieve a direct connection between various dc bus voltages. A multiwinding dual active bridge (DAB) structure is proposed to be used in that occasion for a dynamic simulation platform. For high voltage side, automatic voltage balance and power-sharing balance can be achieved automatically without mandatory power sharing control. To deal with wide voltage conversion gain application, a multimode modulation with peak current value optimization is proposed. It has the tolerance for the short circuit operation under dc fault. The power transmission can be suppressed rather low during the fault. A 1 kV/1.6 kV 10 kW DCT has been fabricated and applied in a dynamic simulation platform. The bidirectional power flow capability together with the short circuit fault tolerance has been verified through the test.

Backup protection method based on multi-interval information in low voltage distribution network of high proportion of renewable energy system

With the large number of renewable energy power sources such as wind power and photovoltaic power connected to the grid, the power system is becoming more electronic. The fault characteristics of renewable energy power sources and synchronous generators are obviously different, mainly manifested as the characteristics of short circuit current amplitude limitation and frequency offset, etc. The traditional protection based on the fault characteristics of synchronous generators may have adaptability problems. For a high proportion of renewable energy system, the short circuit current in the low voltage distribution network is greatly affected by the operation state of the renewable energy power sources after the short circuit fault occurs, which leads to the difficulty of the traditional three-stage overcurrent protection setting and coordination. In order to ensure the safe and stable operation of the system, this paper proposes a new backup protection method for low voltage distribution network. This method uses the information of the electric parameters on the high voltage side and low voltage side of the transformer. Firstly, the modulus sum of the voltage sampling value, the equivalent distance between the fault point and the protection installation place, and the modulus sum of the current sampling value are calculated. Secondly, the direction is judged by the criterion of the voltage direction element and the criterion of the distance protection element based on the multi-interval information. Finally, the maximum current branch of the low voltage distribution network is selected as the fault branch by the overcurrent element based on multi-interval information, so as to trigger the circuit breaker trip and isolate the fault. The low voltage distribution network model of the high proportion of renewable energy system is built on the PSCAD/EMTDC electromagnetic simulation platform. The simulation results show that the proposed protection method can reliably achieve fault removal by using multi-interval information, and has strong adaptability to the high proportion of renewable energy system.

Strength Analysis of Tubular Oil to Oil Cable Box Power Transformer GIS 60 MVA in the Case of Voltage Arc Energy Using the Finite Element Analysis Method

The power transformer is an essential part of the electrical distribution system because it must be reliable and safe from fire and explosions, one of which is brought on by internal oil arching that results in overheating. In general, on the high voltage side of the transformer, there are power transformers fitted with tubular oil to oil cable boxes for the demands of the client. The tubular oil to oil cable box is installed to reduce space in substations with limited space as well as to boost safety against weather and pollution at the high voltage terminal section of power transformers. The goal of this investigation is to determine the cause of an explosion that occurred in a tubular oil-to-oil cable box on the high voltage side of a Gas Insulation System (GIS) type power transformer with a power of 60 MVA and a voltage rating of 150/20 kV. For all parties involved, this incident will serve as a lesson about what to avoid doing in the future with similar power transformers. In this study, we will use a qualitative method with Finite Element Analysis (FEA), which takes samples and data of a Gas Insulation System (GIS) power transformer on the high voltage side, especially in the analysis of tubular strength. This power transformer has a voltage rating of 150/20 kV and a power of 60 MVA. Applying normal pressure to a pressure that could harm the tubular oil to oil cable box can prevent damage. The tubular oil to oil cable box is built of SS400 material and has an 8mm thickness so that it can be determined how robust it is.

Research on Control Strategy of Solid State Transformer Based on Improved MPC Method

As the core equipment of smart grid, solid-state transformer (SST) needs to have good power quality regulation capability under grid side faults and load side faults. To ensure that the high-voltage side converter works at unity power factor and smooth DC voltage. At the same time, the three-phase output voltages need to have high steady-state accuracy and strong anti-interference capability. In this paper, an improved model predictive control scheme was proposed. First, a theoretical modeling analysis of the high and low voltage side of a solid-state transformer was performed. Then, power and voltage prediction models are developed for the high-voltage side converter and low-voltage side converter of the SST, respectively. Three voltage vectors are selected for prediction by the cost function, and the equivalent voltage vector in the two-phase stationary coordinate system is synthesized and modulated to control the converter. Finally, simulation was conducted to compare and analyze various operating conditions of SST under different control schemes. The control effect of solid-state transformer under improved model predictive control is better than traditional finite control set model predictive control and proportional integral control, which can better regulate power quality.

Effect of radical scavenger on electrical tree in cross‐linked polyethylene with large harmonic superimposed DC voltage

AbstractIn this study, three kinds of radical scavenger Chimassorb 944, Tinuvin 622, and Tinuvin 770 are used to suppress the growth of electrical trees in cross‐linked polyethylene (XLPE) with transient superimposed voltage under the temperature gradient. The tree morphology, tree length, accumulated damage, and time to breakdown are used to investigate the effect of radical scavenger on the electrical treeing process. It is found that under the temperature gradient caused by the temperature rise on the high voltage side, only Tinuvin 622 can always suppress the electrical tree as the temperature gradient rises. Under the temperature gradient caused by the temperature rise on the ground side, the three radical scavengers can all suppress the electrical tree. The breakdown of electrical tree exhibits the strong DC polarity dependence. Meanwhile, energy levels of these three radical scavengers are calculated through quantum chemistry, and the results indicate that radical scavengers have greater electron affinity, smaller ionisation energy, and smaller energy gap than XLPE. According to the surface potential decay test results at 60°C, it is found that all three radical scavengers can introduce deep traps. The different performances of radical scavengers under different temperature gradients and voltages are determined by the trap distribution characteristics, the molecular structure and chemical reaction of the additives themselves. It is concluded that Tinuvin 622 has potential for use in high voltage direct current XLPE cable application.

Effect of electrical erosion on composite insulator core rod under acidic environment

Silicone rubber housings crack under the combined internal and external factors and expose the core rod. The exposed core rod deteriorates or breaks under electric fields and electrical current. Based on the main characteristics and mechanism of the core rod corrosion, this paper performs an electrical erosion test in an acid-fog environment and analyzes the core rod's macroscopic results and physicochemical properties. The results indicate the following. First, the discharge on core rod during the electrical erosion process contains four consecutive stages. The electrical current, partial discharge, temperature increase, and macroscopic morphology show regular changes at different stages. The arc usually occurs in the carbonization channel, making the channel have the maximum heat. The area of the core rod heating constantly changes but mainly at the high-voltage side and presents as a regional temperature increase. Second, the fracture of glass fiber and ablation of epoxy resin matrix are related to the continuous action of nitric acid. The generation, expansion, and release of pyrolysis gas form bubbles and pores on the epoxy resin matrix. Erosion and pyrolysis cause the loss of the epoxy resin matrix. Third, the combined effect of electrical current, nitric acid, and heat damage the core rod axially and radially. The study provides support for the subsequent core rod deterioration and fracture mechanism.

An Approach to Performing Stability Analysis for Power Transformer Differential Protection: A Case Study

Differential protection normally detects short circuits and ground faults in the windings of a power transformer and its terminals. Inter-turn faults refer to flashovers among the electrodes that arise only in a similar type of physical winding. Inter-turn faults can be examined when the adequate sum of turns is served as short-circuited. In electrical protection, it is difficult to detect inter-turn faults. An inter-turn fault of small magnitude is based on the limited number of turns that resultantly provide a large quantity of current. Due to this reason, protection that comes from the differential scenario possesses a higher degree of sensitivity without causing unwanted operations during external faults. In this paper, a protection-based stability method is proposed whereby external voltages are applied at the low-voltage (LV) side of the transformer while keeping the high-voltage (HV) side short-circuited. This was conducted using a three-phase power transformer (rating: 100 MVA, 380 kV/13.8 kV) at SWCC Shoaiba Power Plant, Saudi Arabia. In this work, differential protection (87T) and high-impedance differential protection for HV-restricted earth faults (REFs) were verified by creating In-Zone and Out-Zone fault conditions to ensure current transformer (CT) circuits and tripping logic. All of the IEDs, protection, and control schemes involved were designed by ABB. This method verifies protection stability for power transformers by implementing differential protection (87T) and high-impedance restricted earth fault (64HV) schemes through creating In-Zone and Out-Zone fault conditions.

Study on Short-circuit current interruption characteristics of Double-break fast vacuum circuit breaker within the minimum arcing time

It is significant for circuit breakers to expel a short-circuit fault in a short time for the new power grid with high proportional photovoltaic or wind power plants to achieve high transient operating stability. Compared with a traditional circuit breaker, a fast vacuum circuit breaker (FVCB), operated by an electromagnetic repulsion actuator, has an extremely shorter opening time and lower deviation in their opening times. This potentially contributes the FVCBs to interrupting a short-circuit fault current in the first half-wave when introducing a controlled switching technology. Meanwhile, it is beneficial to improve the short-circuit current breaking capacity of the FVCBs by adopting series-connected double-break circuit breaker technologies. This paper aims to explore short-circuit current interruption characteristics of a double-break FVCB within the minimum arcing time. We performed a series of short-circuit current interruption tests in a Weil synthetic test circuit. Two series-connected FVCBs and independent single-break FVCBs were used. Three different kinds of opening velocities were adopted for each of the FVCBs. Results showed that the increase in opening velocity resulted in both a decrease in the minimum arcing time and the critical arc extinguishing contact gap, whether for the single-break or the double-break interruption tests. The ratio of the shared voltage on each of the series-connected FVCBs was about 54 (for the break on the high-voltage side) and 9 (for the break on the low-voltage side). Taking the break with a shorter opening time as a break in the high-voltage side could reduce the minimum arcing time and the critical arc extinguishing contact gap. At the same opening velocity, the sum of the critical arc extinguishing contact gap of the two breaks in double-break interruption tests was significantly higher than twice the critical arc extinguishing contact gap in the single-break interruption tests. This could be caused by the fact that in double-break interruption tests, after the breakdown of one break, even if the break restored insulation, could not share the transient recovery voltage (TRV) before the break broke down. In addition, the sudden drop of TRV without breakdown occurred for the break on the high-voltage side might correlate with the micro-discharge of the vacuum gap. The interaction of the charged particles with the electrode surface would be responsible for the formation and evolution of the micro-discharge in some successful interruption cases.

A Solid-State Bipolar Pulse Power Generator for Dielectric Barrier Discharge Applications

This article provides a distinctive unipolar/bipolar high voltage pulse power generator (PPG) architecture that produces exponential waveform with a rise time in the range of nanosecond. The setup is fit for dielectric barrier discharge applications, in practice. In this work, a push-pull circuit with a three-winding transformer is utilized on the charging side, as well as a quasi-resonant between the transformer leakage inductance and the capacitor on the discharging side to boost the voltage gain. As a result of the two windings on the primary side, and the bidirectional switches on the high voltage side, the tool offers either unipolar or bipolar high voltage pulses from a relatively low dc source. In addition to accomplishing the high gain objective, the effort focuses on lowering the number of circuit elements in contrast to previously studied topologies. The generator functions in discontinuous conduction mode, which enhances system efficiency by allowing complete positive and/or negative waves to be generated while lessening conduction losses. The pulse generator operation intervals are described in depth. The simulation results will demonstrate how this topology outperforms the prior models. Moreover, a 1000-V experimental prototype is also implemented to validate the projected PPG-enhanced properties in real-time.

Inspection and analysis of a 500kV main transformer excessive acetylene failure

B phase of the no 2 main transformer of 500 kV XX substation was suddenly stopped on August 20, 2021 due to acetylene and an abnormal sudden increase. Acetylene reached 10.5uL / L, far exceeding the acetylene attention value of 1.0uL / L, and there is a trend of continuous growth. An internal inspection was carried out after the power outage, and no obvious abnormality was found. Then it was sent to factory for de-oiling and drying. Inspection found the following problems: 1. The upper and lower iron yoke clamps of main transformer and their pull straps, the fourth pull strap shielding cover from the left of lower clamp on the high pressure side was found to be abnormal; 2. Check the iron core silicon steel sheet, found that there are about 15mm burrs between the iron yoke (high-voltage side) core stages under the side post position; 3. There is a black mark at the connection part of low-voltage lead and the casing oil, but no signs of discharge or overheating are seen; 4. Check that there is no obvious abnormality in other parts of the body, such as insulation, lead wires, and electrical shielding. Combined with the inspection situation, analyze specific causes of the failure and propose follow-up improvement suggestions.