Ultra-high Voltage Systems Research Articles

Optimal planning energy storage for promoting renewable power consumption in the urgent situation of UHV systems

AC/DC hybrid ultra-high voltage (UHV) transmission network is an effective way to deliver large scale renewable energy. Unfortunately, the power transmission capacity is significantly restricted due to guaranteed transient stability. Energy storage systems (ESS) are regarded to be the most flexible means to enhance transient stability. However, optimal planning of ESS for UHV stability is challenge because it involves differential equations. For this, this paper firstly proposes the mathematic formulations for optimal planning of ESS with UHV transient stability. The proposed model considers the DC blocking fault that is simulated by component switching equivalent processing. Mathematically, the proposed model belongs to the nonlinear, mix-integer, discontinuous differential constrained programming. The existing simulation and numerical methods are challenging to solve the proposed model directly. For solving this problem, an effective method is proposed, combining with the methods of implicit trapezoidal integral method, sensitivity analysis method and interior point method. The simulation results show that ESS can enhance the transmission efficiency of UHV transmission lines. From the analysis of delivery capacity, voltage distribution, the generator rotor relative swing angle and angular velocity, the technical benefit can be maximized by optimal planning ESS with the proposed method.

Noise suppression for weak current measurement based on neural-network-assisted UHV FOCS

Weak current measurement plays an essential role in the power grid systems, which remains challenging to be achieved with fiber optic current sensor (FOCS) due to ambient and internal noise. In this work, noise suppression method based on back propagation neural network (BPNN) is proposed to calibrate FOCS which is originally applied to ultra-high voltage (UHV) systems so that output accuracy can be improved when it is utilized to detect weak current. The output errors induced by noise, the feature of FOCS output signal and the reason for choosing BPNN are deeply analyzed. Performance of BPNN with different parameters is also investigated and results show that BPNN with 2 hidden layers and 3 neurons meets the requirements of high efficiency and high accuracy in our application. Then, 15750 groups of FOCS output data are collected under different temperature and utilized for network training. With the well-trained BPNN, weak current as low as 0.1 A is successfully detected by UHV FOCS when temperature varies from -30 °C to 70 °C, and ratio error (RE) is limited between -0.2% and 0.2%, showing a better performance on noise elimination than traditional Gaussian filter and Fourier filter.

Working principle and testing technology of circuit breaker closing resistor for 1100kV GIS

The closing resistor can suppress the operation overvoltage and release the energy caused by the overvoltage generated by the power system when switching the line. It is widely used in the ultra-high voltage system AC system. Due to the complicated structure of the circuit breaker with the closing resistor, and the closing resistance of the field is placed in the sealed casing or GIS arc extinguishing chamber, regardless of whether the circuit breaker is in the closing or opening state, the closing resistance is usually not connected. The circuit breaker of GIS is more complicated than the open circuit breaker, especially the circuit breaker structure of 1100kV GIS which is the most complicated, and the maximum resistance of the closing resistor and the closing time of the closing resistor are tested. To understand this, it is very important to understand the working principle and testing technology of the ultra-high voltage circuit breaker. How to carry out preventive tests at the substation site and obtain the technical parameters of the closing resistor has become a key technical problem to be solved urgently. This paper expounds the structure of the ultra-high voltage circuit breaker, analyses the working mechanism of the closing resistor, and gives the method of on-site pre-test, combined with the Yucheng UHV ZF27-1100 (L) GIS circuit breaker to explain and give a case of on-site testing of the pre-input time of the closing resistor was made.

Multi-variable dependent forecast algorithm for predicting secondary arc characteristics in UHV systems with HRPC

Hybrid reactive power compensation (HRPC) combines step-controlled shunt reactors and series compensation, and will be employed in ultra-high-voltage (UHV) power systems. The single-phase auto-reclosure characteristics of secondary arcs in systems with HRPC require further investigation. In this paper, both the arc-recalling voltage and subsidiary variations in arc current are investigated with and without HRPC. The frequency components of the secondary arc current and variations in arcing time are analyzed for various influential factors, such as the neutral reactor, arc resistance, fault location, degrees of compensation of HRPC, and the length of the transmission line. The non-dominated sorting genetic algorithm II (NSGA-II) and support vector machine regression are combined to create a multi-variable dependent forecasting algorithm to predict the characteristics of the secondary arc in UHV systems with HRPC. This paper provides a theoretical reference for optimizing the parameters of HRPC, and for developing adaptive auto-reclosure schemes and protection equipment.

Estimation of Switching Surge Flashover Rate of a 1200 kV UHVAC Transmission Line due to Switching Overvoltages

The rapid increase in transmission voltages to fulfil higher demand of the transmitted power yields to put the switching surges as the governing factor in the insulation design process for EHV and UHV systems. Contemporary methods of insulation coordination utilise probabilistic approach and statistical methods particularly in the case of ultra-high voltage systems. This paper determines the statistical distributions of energization, re-energization, fault initiation and fault clearing overvoltages of the 1200 kV system modelled in PSCAD. Decisive figures like mean value, standard deviation and 2% statistical overvoltage values of switching overvoltage are estimated using randomness of the instants of closing with 526 operations for line energization and re-energization, and randomness of faulted phase, fault location and fault times with 3 faulted phases, 8 fault locations and 21 fault timings, 504 simulations for fault initiation and fault clearing. Ascertaining the investigations from the above simulations the stress on the towers is estimated. From the stress and insulation strength of the tower a systematic approach to calculate the switching surge flashover rate (SSFOR) is presented.

Research and application of UHV power transmission in China

The power demand increases rapidly in China; however, the areas of huge power demands are of long distance from most areas of abundant energy resource in the country. Therefore, China put in great effort to develop ultrahigh voltage (UHV) power transmission systems to optimise its energy allocation. This includes (i) systematically developing key technologies such as overvoltage suppression, external insulation design, and electromagnetic environment control, and (ii) developing key equipment such as UHV alternative current (UHVAC) transformers, circuit breakers, and series compensated equipment, and UHV direct current (UHVDC) converter transformers, smoothing reactors, converter valves, and DC transmission control and protection systems. Eight AC UHV projects and 11 DC UHV projects have been built in China, which play an important role in the optimal allocation of energy. Plus there are one more UHVAC and three more UHVDC transmission projects in construction, while UHVAC gas-insulated lines will be applied in the UHVAC line crossover the Yangtze River and the ±1100 kV UHVDC power transmission technology is in development. Here, the development and application of UHV power transmission technologies in China are described, some main challenges the UHV projects encountered are discussed, and experiences obtained from the operation of UHV systems are introduced. It is concluded that China obtained mature experience in developing, constructing, and operating UHV systems and successfully realised long-distance, large-capacity power transmission, and the UHV power transmission technology has become an important measure for energy allocation in large areas.

Influence of Hybrid Reactive Power Compensation on the Secondary Arc of Ultra-High-Voltage Transmission Lines (May 2018)

Hybrid reactive power compensation (HRPC) which consists of series compensation and a stepped-controlled shunt reactor will see use in China's future ultra-high-voltage (UHV) transmission system. As a result, some new characteristics of the secondary arc during a single-phase-to-ground fault on UHV transmission lines with HRPC may be raised. In this paper, the secondary arc characteristics during the single-phase auto-reclosure (SPAR) operation have been simulated with regard to the UHV AC system installed with HRPC. Based on the UHV system model with HRPC and its secondary arc model, the influence mechanism of HRPC on secondary arc current is theoretically derived with the simulated waveforms and the Fourier analysis. Besides, the relationship among the secondary arc current, the arcing time with different HRPC compensation degrees, neutral reactance, as well as the grounding fault locations have been investigated in detail. Finally, an improved time sequential control strategy of bypass breaker and SPAR has been put forward. The research presented in this paper will provide necessary theoretical basis and technical support for the future application of HRPC in UHV transmission lines.

Application of smart computing techniques in cost optimization of 1200 KV autotransformer

This paper aims to identify the scope for cost optimization in a 1200 kV autotransformer manufacturing through the use of smart computing techniques. A typical Ultra High Voltage (UHV) power transmission system consists of components like Autotransformers, Capacitive voltage transformer, Surge arrestors, Circuit breakers, Isolators, Bus post insulators and hardware of transmission line structure. Through various UHV projects running worldwide, technical feasibility of UHV system has already been proved but in view of cost optimization of overall system, research is going on. Autotransformer is a main component of UHVAC transmission system and needs to be critically investigated to find a room for most economic and commercial viable manufacturing. Use of Finite Element Method (FEM) based simulation has proved to be an effective tool for carrying out design of insulation system and other structural design prepared by Indian manufacturers. A questionnaire is developed for analyzing the scope for cost optimization of autotransformer on the basis of their complexity, design constraints, dielectric requirements, electrical stress, testing facility etc. The statistical analysis of data obtained, can be processed through statistical analysis software(s) available to identify the scope for cost optimization, efficiency improvement and cost saving in manufacturing.

Influence of profile on the pollution performance of cap-and-pin insulators-an experimental study

Transmitting power by extra-high voltage and ultra-high voltage systems becomes obligatory with the rapid increase in power demand by industrialization and urbanization. Insulators in the transmission system play a significant role in maintaining the reliability of the network by providing the required mechanical support and electrical insulation.

Transient measured impedance‐based protection scheme for DC line faults in ultra high‐voltage direct‐current system

DC line fault is a major threat to the security of ultra high-voltage direct-current system due to complex terrain and terrible weather conditions over long distances. This study proposes a DC line backup protection method on the basis of transient measured impedance (TMI), which is defined as the ratio of transient voltage and transient current at both sides of DC line. The characteristics of TMIs are theoretically analysed for DC line faults and external faults, and these characteristics are used to construct a protection scheme. Since the TMIs only depend on the system structure, the proposed method is able to identify the faults correctly, unaffected by the fault resistance and DC line attenuation. A selection method for the faulted line is also proposed based on the ratio of transient component on the bipolar lines. Comparison with the transient-component-based protections and conventional protections demonstrates the promising performance of the proposed protection. Numerous simulations on PSCAD/EMTDC platform and the tests with field fault-recording data have validated the effectiveness of the proposed protection scheme.

Electromagnetic Calculation of a 35 kV/3.5 MVA Single-Phase HTS Controllable Reactor With Field–Circuit Coupled-FEM

A controllable reactor is able to compensate a flexible reactive power in a long-distance and an ultrahigh-voltage system. A high-temperature superconductivity controllable reactor (HTS-CR) is a potential equipment in the power system. A method to accomplish the electromagnetic calculation of the HTS-CR is proposed, wherein the field-circuit coupled finite-element method (FCC-FEM) is introduced. Test results of the steady-state current of all the windings of an HTS-CR prototype is compared with the calculation results, which proves the reliability of FCC-FEM. Based on FCC-FEM, the accuracy of the four-probe method, which is used to measure the dc critical current of HTS winding with the criterion 10 -6 V/cm, is studied. Contrary to the four-probe method, a new method based on FCC-FEM, which has better accuracy, is designed to calculate critical current. With FCC-FEM, distributions of the current in the parallel-connected HTS tapes and the ac critical current of the HTS windings for a 35 kV/3.5 MVA single-phase HTS-CR are presented.

Design of Cryogenic Cooling System of a 35-kV/3.5-MVA Single-Phase HTS-Controllable Reactor

In order to compensate flexibly reactive power, it is necessary to develop a controllable reactor in long-distance and ultrahighvoltage systems. A 35-kV/3.5-MVA single-phase high-temperature superconductivity-controllable reactor (HTS-CR) is being developed. The control windings of the HTS-CR are made of HTS. The closed-cycle cooling method with the subcooled liquid nitrogen (LN) flowing in the cryogenic cooling system of the HTS-CR is introduced to cool down the HTS windings. The number and the layout of both the inlet pipes and the outlet pipes of the cooling system, which has great effect on the temperature rise of the HTS winding, are studied. Furthermore, the flow velocity of the subcooled LN of the inlet pipes is also investigated, which discloses the optimized structure design of the cryogenic cooling system. The study here is believed to provide some references for developing HTS-CR with larger capacity in the future.

Failure analysis of CuW/CuCrZr contact materials in capacitor bank switch

The type test of CuW/CuCrZr arc contact used for ultra-high voltage system capacitor bank switch was conducted. Through the comparative analysis of morphologies, compositions, properties and microstructures of the contact materials before and after failure, it can be found that the abnormal open of moving arc contact and the arc erosion and mechanical wear of static arc contact are the major reasons affecting the arc contact failure. The failure mechanism and main affecting factors were investigated in detail. The results showed that arc erosion and mechanical wear are the two main factors that affect the service lifetime of arc contacts used for ultra-high voltage transmission lines and cannot be ignored. Therefore, improving the arc erosion resistance and mechanical wear resistance simultaneously is the key to prepare the arc contact materials with longer lifetime.

Impact of the EHV Power System on Geomagnetically Induced Currents in the UHV Power System

Earlier calculations of geomagnetically induced currents (GICs) in Chinese power grids have mainly concentrated on the highest voltage-level system whose geomagnetic risk is considered to be the largest, thus ignoring secondary voltage-level systems. With the 1000-kV system being newly added to China's power system, it is significant to figure out the interaction between GIC in the 500 kV [extremely high voltage (EHV)] system and GIC in the 1000 kV [ultra-high voltage (UHV)] system. Based on the North China-Central China-East China Power Grids, this paper establishes a single-voltage grid by only considering the 1000-kV system and a dual-voltage grid by considering the 1000- and 500-kV systems and investigates GIC in these two grids by developing their GIC “Full-node models.” The impact of the 500-kV system on GIC in the 1000-kV system is analyzed. GIC risks in the UHV and EHV systems are assessed by comparing calculated GIC data with monitored values of GIC. The results show that the impact of the 500-kV system on GIC in the 1000-kV system is obvious, both the EHV and the UHV grid have a high GIC risk. So calculating GIC in the UHV system and in the EHV system should utilize GIC modeling methods for multivoltage power grids.

Design of Superconductivity Windings of a 35-kV/3.5-MVA Single-Phase HTS-Controllable Reactor

In order to compensate flexibly reactive power, it is necessary to develop a controllable reactor in long-distance and ultrahigh-voltage system. High-temperature superconductivity-controllable reactor (HTS-CR) is a potential equipment in power system. A 35-kV/3.5-MVA single-phase HTS-CR is being developed. The HTS windings of the HTS-CR are made of double pancakes that are wound with HTS tapes connected in parallel. The relationship between the number of double pancakes and the number of HTS tapes, which has great effect on the current of each HTS tape, is studied. Furthermore, the solution that reduces the ac loss of the HTS winding is also investigated, which discloses the optimized design parameters and the calculation results of the HTS windings. The study here is believed to provide some references for developing HTS-CR with larger capacity in the future.

Long front time switching impulse tests of long air gap in UHV projects at altitude of 2100 m

The past and recent investigations show that front time of switching overvoltage waveforms in Ultra High Voltage (UHV) system of long distance transmission are mostly in long wave front time exceeding 1000μs. The long front time of switching impulse voltage has been deviated from the critical front time. This paper is involved with breakdown characteristics and external insulation design under long front switching impulse voltage in UHV system. All the Tests were performed on an outdoor UHV test site at altitude of 2100 m, China Southern Power Grid Co. Ltd. (CSG). Breakdown characteristics of rod-plane and actual size tower-conductor in /spl l.chemo/800 kV UHV DC transmission line are obtained under wide front time switching impulse voltages from 100 to 2350 μs. From the comprehensive studies of such tests, it is proposed that U50 of front time exceeding 1000 μs increases 20% compared with the results of standard switching impulse (250/2500 μs) tests and standard deviations of U50 is 6% in UHV transmission system. Finally, an air gap insulation design procedure in conductor-tower of UHV transmission line at altitude of 0-2000 m is presented. Long front time switching impulse strength of conductor-tower gap is proposed into selection by reference in UHV transmission line insulation design.

Development of a Leakage Flux-Controlled Reactor

In order to flexibly compensate reactive power, it is necessary to develop a controllable reactor in long distance and ultrahigh voltage system. A leakage flux-controlled reactor (LFCR) is developed in this paper. The operation principles of LFCR are discussed in detail, and a 380 V single-phase LFCR prototype is developed. Relevant experiments have been done to test its performance. The results of the experiments indicate that LFCR has no harmonic and the reactance of LFCR is stable. Also, LFCR with two control windings can realize four kinds of reactance adjustment rates: 1.94%, 75.99%, 90.53%, and 99.05%.

Lightning Impulse Corona Characteristic of 1000-kV UHV Transmission Lines and Its Influences on Lightning Overvoltage Analysis Results

The lightning overvoltage is one of key issues in designing the insulation of transmission line and power apparatus. Usually the influence of the impulse corona is ignored in numerical analysis of electromagnetic transients in power systems due to the lack of experimental data. But the relationship between the overvoltage and the insulation size of 1000-kV ac ultra-high voltage (UHV) power systems enters the nonlinear region, and a small decrease in overvoltage will lead to a high reduction in insulation size and bring huge economical benefits. So fully understanding the corona characteristics of the 1000-kV UHV transmission line is helpful to understand the actual wave process and realize a suitable insulation design of the 1000-kV UHV system. Experiments on the lightning impulse corona characteristic of bundle conductors for 1000-kV transmission lines are carried out in the China UHV ac test base and the impulse corona Q-V curves of different bundle conductors are obtained. The results show that the impulse corona leads the increment of the capacitance of the conductor. Analysis on the Q-V curves shows that the occurrence of the positive impulse corona has a time delay after the lightning impulse voltage exceeds the corona inception voltage and the delay time is a tenth of a microsecond. At the end of this paper, a simulation model for corona based on the empirical formulas of Q-V curves fitted from the test results is proposed, and is applied to the overvoltage analysis of 1000- kV transmission line and substation. The analysis results show the lightning impulse corona has a large influence on the lightning overvoltage of the transmission line and substation.

Equivalent Waveform Parameters of Switching Overvoltages in UHV Systems

The standard waveform of switching impulse is important for insulation tests of air gaps as well as external insulation and inner insulation of power apparatus in the power system, which should be consistent with the waveforms in actual power systems. Different definitions of waveform parameters of standard switching impulse are discussed in this paper, and a definition based on breakdown characteristics of air gaps is suggested to obtain the front times. Waveform parameters of switching overvoltages in two 1000-kV ultra-high voltage systems in China are studied and their statistical regularities under different operation conditions are analyzed. Based on the breakdown characteristics of air gaps and the concept of equivalent insulation performance, the typical front times in the system are obtained by a weighted average method.

Research on lightning performance of AC/DC hybrid transmission lines on the same tower

Rapid expansion of power system and the limitation of transmission corridor resources have indispensably promoted the development of AC/DC hybrid transmission lines on the same tower under special situations, especially for ultra-high voltage systems being established in China. A hybrid tower model incorporating a double-circuit AC transmission and a bipolar DC transmission is proposed in the study. The lightning performance of AC/DC hybrid transmission lines is investigated based on electro-magnetic transient program (EMTP) and leader progression model, taking into account the operating voltage of transmission lines as well as the competition of multiple upward leaders. The impact of tower's height, footing resistance, arrangements of ground wires and ground obliquity on lightning protection performance is also analysed. In accordance with the ± 800 kV bipolar DC and the 500 kV double-circuit AC transmission lines on the same tower, the results show that the back flashover and the shielding failure is relatively serious for a 500 kV AC transmission line, which should be paid more attention to. Finally, several methods that may be used to improve lightning protection performance of the AC/DC hybrid transmission lines are presented, such as reducing the tower's height, using a negative protection angle and installing additional ground wires.