Influence of AC voltage on the positive DC corona current pulses in a wire-cylinder gap View Document

In the development of hybrid HVDC and HVAC transmission lines, the study of radio interference is an important issue. Positive corona current pulses from high voltage transmission lines are the main source of radio interference. In this paper, the design of a wire-cylinder gap electrode system is presented to study the influence of AC voltage on the characteristics of positive corona current pulses. The study shows that the mode of the current pulses is different from that of either DC or AC corona discharge. Waveform parameters of the pulses, such as rise time, half wave time, duration time, repetition rates, average amplitude, and time intervals of secondary pulses are all statistically analyzed in this study. The empirical formulas for the repetition rates with different AC voltages are presented. A theoretical explanation based on an ion cloud model is given to reveal the mechanism behind the influence of AC voltage on positive corona discharge. The experimental results could provide some references for the prediction of radio interference from hybrid AC/DC transmission lines

Numerical investigation of the high pressure selective catalytic reduction system impact on marine two-stroke diesel engines

After-treatment systems using the selective catalytic reduction (SCR) technology have demonstrated a potential to reduce the nitrogen oxides (NOx) emissions from marine engines by more than 90% with its most typical configurations being the high pressure system (SCR-HP) and the low pressure system (SCR-LP). This study aims to investigate the impact of the SCR-HP system on a large marine two-stroke engine performance parameters by employing thermodynamic modelling. A coupled model of the zero-dimensional type is extended to incorporate the modelling of the SCR-HP system components and the control bypass valve (CBV) block. The CBV control system is modelled based on the exhaust gas minimum temperature set point, which is considered a function of the sulphur content and the exhaust receiver pressure. This model is initially validated against experimental data and subsequently employed to simulate several scenarios representing the engine operation at both healthy and degraded conditions considering the compressor fouling and the SCR reactor clogging. The derived results are analysed to quantify the impact of the SCR-HP system on the investigated engine performance. The SCR system pressure drop and the cylinder bypass valve flow cause an increase of the engine specific fuel oil consumption (SFOC) in the range 0.3 to 2.77 g/kWh. The thermal inertia of the SCR-HP system is mainly attributed to the SCR reactor, which causes a delayed turbocharger response. These effects are more pronounced at low engine loads. This study supports the better understanding of the operating characteristics of marine two-stroke diesel engines equipped with the SCR-HP system and quantification of the impact of the components degradation on the engine performance. Furthermore, it provides insights for the effective shipboard operation of these engines and the SCR-HP system

Experimental Study of Synergism in N2 and SF6 Gas Mixtures

Although sulfur-hexafluoride (SF 6 ) has been used as an insulating medium in various power system applications it has been shown that its greenhouse effect is significant. Due to this effect throughout the world, many countries have already issued restrictions on the usage of SF 6 , which may adversely affect various apparatus and systems presently being used in power industry. In order to solve the problem without a delay studies have been undertaken to find a replacement gas for pure SF 6 so that the existing insulation of a power apparatus or a system can be modified. Among many alternatives, nitrogen and SF 6 (N 2 + SF 6 ) mixtures seem to be the simplest and the least expensive alternative due to N 2 being inert, abundant and renewable. Because SF 6 is an electronegative gas, the synergism may take place in mixtures of SF 6 with N 2 . The synergy in a gas mixture is considered when the value of the dielectric strength of the mixture is higher than the value determined by the linear combination of the dielectric strengths of each component gas in the mixture. Therefore, in this study the degree of synergism is assessed due to electronegativity of SF 6 and the non-linear behavior of this binary mixture in order to determine if there exists an optimum SF 6 concentration

Influence factors and reasonable arrangement of corridor width of ±800 kV HVDC and 750 kV HVAC hybrid transmission lines

With the development of economy and the increased demand of power energy, it is unavoidable that high-voltage direct current (HVDC) and high-voltage alternating current (HVAC) transmission lines are in the same corridor. In order to minimise the land resource and meet the requirements of the electromagnetic environment, it is necessary to accurately predict the hybrid electric field under the hybrid transmission lines. The ±800 kV HVDC and 750 kV HVAC parallel transmission lines of parallel erection are chosen to be analysed. The corridor width with a different separation distance is calculated, and the minimum corridor width under this voltage-level combination is obtained according to the control value of the hybrid electric field recommended at home and abroad in a non-residential area. Increasing the height of DC or AC lines can reduce the corridor width. At a fixed separation distance, the minimum height of the AC line can be obtained when the height of the DC line is set at a certain value. Meanwhile the change of the corridor width is analysed with different heights of DC lines and AC lines. Based on the above analysis, the reasonable arrangement is made

Prediction of switching transient electromagnetic fields in gas-insulated substations

Due to the opening or closing operation of circuit breakers and disconnected switches in gas-insulated substations (GIS), a very strong transient wave process can be generated along the three-phase transmission system and transient electromagnetic field can be radiated from the junction between the GIS enclosure and the high-voltage cables. Based on a kind of hybrid method, the software TEMFGIS is provided to predict the radiated field in time domain. Its validity is verified by the measurement result

Propagation characteristics of audible noise generated by single corona source under positive DC voltage

The directivity and lateral profile of corona-generated audible noise (AN) from a single corona source are measured through experiments carried out in the semi-anechoic laboratory. The experimental results show that the waveform of corona-generated AN consists of a series of random sound pressure pulses whose pulse amplitudes decrease with the increase of measurement distance. A single corona source can be regarded as a non-directional AN source, and the A-weighted SPL (sound pressure level) decreases 6 dB(A) as doubling the measurement distance. Then, qualitative explanations for the rationality of treating the single corona source as a point source are given on the basis of the Ingard’s theory for sound generation in corona discharge. Furthermore, we take into consideration of the ground reflection and the air attenuation to reconstruct the propagation features of AN from the single corona source. The calculated results agree with the measurement well, which validates the propagation model. Finally, the influence of the ground reflection on the SPL is presented in the paper

Statistical analysis of audible noise generated by AC corona discharge from single corona sources

Time-domain waveforms of sound pressure pulses generated by alternate current (AC) corona discharge from single corona sources are measured. The pulse amplitude and time interval of sound pressure pulses in positive and negative half cycles are analysed separately due to large differences of the sound pressure pulses in positive and negative half cycles. The influences of instantaneous voltages on the pulse amplitude and time interval are obtained, and statistical distributions of pulse amplitude and time interval are also obtained. Besides, empirical formulas of the pulse amplitude and time interval are derived by numerical fitting method. Based on the statistical results, stochastic simulation of the sound pressure pulses is conducted, and the validity of the simulated results is given by comparing the frequency-domain results and A-weighted sound pressure level (SPL) with the measured ones. Finally, the stochastic simulation method is used to discuss modulating influence of AC voltage on the tones emission from AC corona discharge. Besides, the influence of the measurement time on the resultant SPL is also discussed on the basis of the stochastic model. In order to insure the SPL independent of the measurement time, the measurement time should be larger than 0.2 s

Analysis of the Influence of Electrostatic Probe on Surface Charge Measurements

The electrostatic probe is among the most popular methods to measure the surface charge. However, the probe influences the surface charge distribution on the sample, which increases measurement error. In this paper, the electrostatic probe system is modeled to study its effect on surface charges on a circular conductor plate using electrostatic field calculation. The influence of the probe on the sample surface charges is found to increase with a decrease in the measurement distance, whereas it has a similar effect on surface charge density at different radial positions. A calibration process is proposed to measure the potential from the induced potential of the probe head. Using the measured potential, we calculate the surface charge density via the method of moments. The measurement error is analyzed when the measurement distance and number of rings are changed. The authors find that the number of rings should be appropriately reduced when the measurement distance increases to ensure the accuracy of the calculations. This research provides theoretical guidance for using the electrostatic probe and establishing a surface charge measurements system

Experimental Study on Spectral Characteristics of Corona-Generated Audible Noise From a DC Conductor

In this paper, spectral characteristics of the corona-generated audible noise (AN) on a dc conductor are studied through a quadrate corona cage structure. To reduce the influence of background noise, all experiments are conducted in a semi-anechoic chamber. Different from previous investigations, time-domain waveforms of corona-generated AN are recorded and the frequency spectra are calculated based on the time-domain waveforms through the calculation method of A-weighted sound pressure level (SPL). The results confirm that the frequency spectra describe different characteristics at different applied voltages and have a close relationship with pulse amplitude and pulse repetition frequency. In addition, the AN from a negative corona discharge contains more high-frequency components than those from a positive corona discharge. The components with the frequency above 400 Hz contribute most to the resultant total A-weighted SPL. The differences of AN from positive and negative corona discharges are interpreted on the basis of the corona discharge physics. Furthermore, some correlation frequency components in the frequency spectrum are found and can be used to calculate the total A-weighted SPL by adding correction values. Finally, the influences of pulse amplitudes and repetition frequency on the frequency spectrum of AN are discussed on the basis of the time-domain stochastic model