Analysis of the Over-criterion Harmonic Current Caused by Background Harmonic Voltages
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Though the customers have installed AC filters,background harmonic voltages may also cause the harmonic current on PCC to exceed the criterion value.The reasons of over-criterion harmonic current are analyzed by testing results of typical project cases,the mechanism of over-criterion harmonic is explained according to harmonic flow analysis,some relevant measure are put forwards to solve the problem.The feasibility and the validity of analysis method is testified by both practice results and theoretic analysis,analysis model may analysis and resolve related issues in engineering.Keywords:
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In this paper, a novel approach is proposed to analyze the harmonic behavior of customers at the point of common coupling (PCC). With the aid of this approach, the origin of harmonic currents at the PCC can be determined. Furthermore, the influence of customers on the harmonic voltage at the PCC can be investigated. To apply the proposed approach, knowing the equivalent impedance of the customer and network as well as the measurement of the harmonic current and voltage at PCC are required as input data. Some aspects of relevance for application of the proposed approach in practice are discussed in this work as well. In this regard, the stochastic nature and asymmetry of harmonics are addressed. The proposed paper lays the foundation for the revision of harmonic evaluation methods in the current standards.
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The excessive use of semiconductor components in the grid brings great challenges to the power quality of the grid. It is quite significant to estimate the harmonic parameters of multiple harmonic sources in power quality evaluation. The existing methods of estimating harmonic parameters are based on the known phase values of harmonic voltage and harmonic current. However, it is very difficult to measure the phase information of harmonic voltage and harmonic current in practice. The phase of artificial structure will inevitably cause uncontrollable errors. Based on the linear regression model, this paper deduced a mathematical model for estimating harmonic parameters in power system by using the phase difference between the harmonic voltage at PCC point and each feeder current including the amplitude of them. Compared with other methods, the method proposed in this paper could estimate harmonic parameters without the phase information of harmonic voltage and harmonic current, which solved the problem that harmonic parameters cannot be accurately estimated due to the difficulty of measuring phase information. The analysis results of the measured data verify the effectiveness of the method.
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Nowadays, harmonic active power direction method is the most widely applied method to judge the main harmonic source, but the power direction method is theoretically different from harmonic current superposition principle. In this paper, it has been got that the power direction method and harmonic current superposition principle are different through mathematical analysis, and the precise conditions which make harmonic active power direction method right has been got. On the basis of this, this paper analyzes the applicable area of the harmonic active power direction method. In practical engineering applications, this method does not require the exact values of the harmonic impedances of the two sides. Besides, the easy way to judge the location of the main harmonic source is obtained in this paper, which only depends on the measured angle value of the harmonic voltage and harmonic current at the PCC. It is convenient to the practical application in engineering.
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Concerning harmonic distortions, in addition to traditional matters such as limits, measurements, effects, and others, the importance of establishing ways to determine the responsibility for possible violations of given limits emerges an important challenge. This subject has been considered by researchers and amongst some proposals, one of the most known methodologies for identification of the major source of harmonics consists in determining the direction of harmonic power flow by comparing the fundamental and harmonic power signals. This attempt has been supported by the fact that commercial power quality measurement equipment has the ability to quantify fundamental and individual harmonic powers. Focusing this approach, this paper is aimed at describing the theoretical fundamentals to the method, as well as to present and discuss computational and experimental studies performed for a hypothetical electrical arrangement. The final results are then used to demonstrate the method weakness to fulfill the established needs.
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The current method of harmonic evaluation is comparing the 95% value with the limit value based on GB/T 14549-93<;Quality of electric energy supply harmonics in public supply network>, which can only judge whether the harmonic is exceeded or not. But it cannot evaluate the degree of harmonic pollution. Under the premise of ensuring the power quality of the utility grid as well as taking the different harmonic characteristics of the harmonic source users into account, it is necessary to refine the excessive degree of harmonics out of specification. And, the harmonics evaluation should link harmonic current assessment to harmonic voltage evaluation effectively to get a better assessment result. And a method of using multi-values maybe can be adopted. Therefore, a method of harmonic assessment with multi-values considering the regulation effect is proposed in this paper. The proposed method divides the harmonics influence on power grid into seven grades, which can give the reference to the harmonic mitigation both considering the cost of harmonic governance and the effect of the harmonic mitigation. The result of the case study proves the proposed method is effective and feasible to the harmonic assessment.
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Reproductive simulation of the measurement data by the harmonic analyze is start point to understand harmonic voltage distortion mechanism, but the method to estimate the harmonics distribution among the many utility customers has not been established. In the last decade, harmonic analysis of Japanese middle voltage customer and low voltage customer has been done, the distribution of the fifth harmonic current can be classified middle voltage customer's single phase load, low voltage customer's three phase and single phase load named group A and middle voltage three phase load named group B, and the amplitude per load kW has been investigated from sum of each current vector. By the application of this knowledge, there is a possibility to estimate the harmonic distribution with many harmonic source in the distribution network. In this paper, we developed a harmonic state estimation method with limited measurement condition by using the fifth harmonic current phase angle classification model. Numerical experiments are conducted in seven node distribution network to test the proposed method. The results show that the proposed harmonic state estimation by using the fifth harmonic current phase angle classification model approach can reliably identify harmonic distribution in the change of load conditions, background harmonic voltage phase angle and series inductance of shunt capacitors.
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Harmonic voltages at each bus of a power grid are the combinations of voltage contributions of each harmonic source located in the same grid.Distinguishing the contributions of each main harmonic source to the harmonic voltages of particular buses is very significant for responsibility partition of harmonic pollution.This paper first analyzes and points out that the critical process in harmonic contribution distinguishing is to calculate the harmonic mutual/self impedance between harmonic source buses and the concerned buses accurately.Then a novel non-invasive method for harmonic contribution distinguishing is put forward.With the fast components of harmonic current injected by harmonic sources as the tool,this method extracts the mutual/self impedance between the main harmonic source buses and the concerned buses according to the statistical property of independent random vectors;finally,harmonic vector contributions and scalar contributions of each harmonic source to all concerned buses can be obtained according to the calculated mutual/self impedance.A case study based on the IEEE 14-bus test system was conducted,it shows that the proposed approach is effective to distinguish the harmonic contributions due to all main harmonic sources to the concerned buses,and can largely eliminates the negative effect caused by the harmonic fluctuation of other harmonic sources in contribution distinguishing;moreover,it also provides a new basis for responsibility partition of harmonic pollution.
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Determining the harmonic responsibilities of utility and customer side at a PCC has been a great concern in harmonic research, and utility harmonic impedance is a key factor for assessing harmonic emission levels. When background harmonic is unstable, traditional methods have a large error. A new method is proposed in this paper to assess harmonic impedance based on minimum energy fluctuation. The method can calculate harmonic impedance by constructing an object function which minimizes the background harmonic energy fluctuation, without assuming background harmonic is stable. Simulation analysis indicates that the proposed method is more reliable to calculate harmonic impedance, which can reduce the error caused by the fluctuation of background harmonic.
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This paper describes advanced algorithms for harmonic assessment. The harmonic assessment includes both harmonic analysis, where a electrical network is modelled along with its non-linear loads to determine the harmonic state throughout the system, as well as harmonic state estimation whereby the harmonic state of a system is determined from limited harmonic measurements. In particular the advanced harmonic analysis technique known as harmonic domain (HD) is outlined and the concept of harmonic state estimation (HSE) presented.
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