Measurement And Studies On Power Network Harmonics

Harmonic is probably the most important power quality (PQ) problem affecting industrial and large commercial customers. In this thesis, a brief summary of the harmonic profile of the distributed systems will be presented. Because the proliferation of small nonlinear loads devices has the potential to greatly elevate harmonic distortion levels on distribution and large nonlinear loads such as electric railways are the main contribution to distribution system harmonics. Investments and analysis are studied in this paper. Main contents are shown in following.1) This paper talks about cause of the impact of harmonic distortion power on the apparent power and power factor. To study this harmonic distortion, we performed field measurements on a variety of home appliances. Data acquisition presents the harmonic current level of nonlinear loads and low-voltage network. The field data show that nonlinear loads can be a dominant effect with respect to distortion power and power factor.2) The analysis uses the time domain approach and utilizes the Alternative Transients Program (ATP) to model the harmonic producing devices and three-phase four-wire balanced system. The analysis proves that the models are identical with the realistic distribution systems.3) We also acquired data from a number of 0.4kV distribution systems supplying typical urban loads. The field data show that nonlinear loads can be a dominant effect with respect to distortion power and power factor. A comprehensive investigation on the neutral and phase current distortion levels behind the low voltage transformer have been carried out in the realistic distribution systems. In this paper we also estimate the increase of losses due to non-linear loads. A simple model for calculation harmonic loss is built based on real power network and actual measured data. This paper proposed that reactive power compensation should include traditional reactive power compensation and distortion power compensation when harmonic exists. The net harmonic loss shows the importance of harmonics control and effective measures and charges for harmonic power.4) The measurement I the 10kv network is incorrect because of the resonance elimination reactor or resistor in neutral point of voltage transformer. The data of Dongying substation shows that third harmonic voltage is unusually high. in the mean time the third harmonic current is at the normal level.5) In the past few years the number of loads producing harmonic in the power system has grown. Higher distortion levels can affect the performance of revenue meters and lead to billing inequities between the electrical utilities and industrial customers. Pricing of electric energy is presently based on the value of integral of the load active power P measured by energy meters. At such a pricing, the electric power utilities waste some revenues for the energy delivered to current harmonic generating customers and/or customers causing current asymmetry. This is because the load generated current harmonics and unbalanced currents cause an increase in the active power loss in the distribution system. At the same time, the customers that do not generate harmonics but are supplied with distorted and/or asymmetrical voltage are billed not only for the useful energy but also for the energy which may cause only harmful effects on their equipment. It is shown in the paper that these two disadvantages of the present tariff in single-phase systems could be eliminated if the energy account based on the value of the integral of the active power of only fundamental harmonic, Pâ… , rather than on the integral of the whole active power P was based. These disadvantages could be eliminated in three-phase systems if the energy account based on the integral of the active power of the positive sequence component of the fundamental harmonic.6) A brief summary of power quality monitoring of power system with the electric railway will be presented. This paper proposes detailed description for a layered, distributional energy quality monitoring platform with electric railway. The monitoring platform is based on the distributional software system including control center, substation and communication channel. The platform supervises completely the electrical energy qualities of harmonic, voltage fluctuation, flicker, three phase unbalance, power factor and so on according to the newest national standard. The examination method can be varied according to the GB code revision. Moreover the platform uses layer-built integrated management and embedded internet technology. It also transparently supports many kinds of communication methods. The substation is mainly composed of a Three Phase Power Quality Analyzer which measures power, energy and power quality data on all 3 phases including harmonics, transients, sags/swells, power factor, inrush. It has large color screen with memory, windows software and USB communications. The platform is opened to operators and can be expended to numbers of substations. It used SQL Server2000 database management system. This energy quality monitoring platform has operated very well in Shandong province with electric railway traction substations along the railroad from Qingdao to Jinan.7) The purpose of this thesis is to give the scheme of how the electric railway connected to the power grid. According to the effect of the electric trains and the traction-transformer, technologic measurements are proposed as to increase the harmonic and unbalanced of voltages and injected currents. Also, the points of which we put the test devices are rationally chosen.8) Real-time data acquisition in the Shandong electric power grid show that harmonic voltage around 25th is tremendous higher than it should be with electric railway. To research and analyze this phenomenon, an emulation model of electrical railway is built using the software of ATPDraw in this paper. The emulation model presents the same results as field data. It explains that abnormal harmonic voltage is caused by resonance. The frequency of certain kind harmonic voltage is near to the resonance frequency of power supply network, it will be verified in this paper that the abnormal high harmonics are magnified by the power supply transmission line with certain LC parameters.