Tao Shun

Study on Distribution Reliability Considering Voltage Sags and Acceptable Indices

In modern power systems, more sensitive loads appear. The impact of voltage sags on customers cannot be ignored. With the opening of electricity markets, it is necessary to modify and supplement the reliability indices to consider the influences of voltage sags. In this paper, three supplementary indices are proposed, that is: 1) event time; 2) sag energy; and 3) cost of events. Furthermore, we try to redefine the traditional reliability indices on service. These indices may be useful today and in the future as well. Computer simulation results show that voltage sags do affect reliability indices, which change the value of the third digit of the decimal part of reliability indices. The higher the reliability level is, the larger the impact of sags on reliability is

Comparing transformer derating computed using the harmonic loss factor FHL and K-factor

The transformer losses will increase while it is supplying nonlinear load current. The derating measure for normal transformers is always taken to limit the temperature of windings or top-oil. In this paper, the formulations how to calculate the maximum permissible rms non-sinusoidal load current (Imax) using the harmonic loss factors FHL and K-factor were introduced. The difference of them was also reasoned and a case study illustrated the process of reasoning. The applications of these two factors were discussed too. Imax computed using FHL for normal transformer is more economical because Imax computed using K-factor is more conservative. Based on the actual monitoring data of harmonic component, K-factor is suitable for selecting right transformers in design and FHL for establishing transformers operation capability, therefore there is a harmonic loss margin when the transformers supply non-sinusoidal load.

A new analysis method for compensation strategy of DVR and minimum energy control

The compensation strategy of the dynamic voltage restorer (DVR) has significant impact on the compensation result. A new analysis method which is based on pre-sag load voltage is proposed in the paper. The new method chooses the pre-sag load voltage phasor as the reference phasor and put the centre of the limit compensation voltage circle at the terminal of the system voltage phasor. It not only has definite physical concept but also is convenient to determine the compensation range. More importantly, it could be applied to unbalanced load. The basic theory and process of the new method was explained and compensation strategy of the three-phase DVR was analyzed. Using the new method, the minimum energy compensation strategy was realized through the simulation when unbalanced voltage sag happened in the case of the unbalanced load. The conditions for zero active power compensation was also analyzed which could prolong the compensation time of DVR. The simulation result shows the correctness of the theoretical analysis.

Synthetic grading evaluation of power quality based on cask principle

In a liberalized electricity market, it is not surprising that different customers need different power quality level with different price. So how to define a synthetic grading evaluation of power quality (PQSGE) became a concerned problem. In this paper, based on four electromagnetic compatibility (EMC) levels, all individual indices of power quality(PQ) were divided into five classes (Class I -Class V); On this basis, a novel PQSGE method based on cask principle was represented, which is easily understood and manipulated by operators, customers and regulators. A case study based on actual monitoring data of PQ showed that the overall quality levels of the assessed site qualitatively reflected the levels of electromagnetic environment and also expressed the characteristics of loads served by the site. The PQSGE can work together with the market mechanism of future electricity market to promote utilities and end-users to improve the overall level of PQ, reduce the whole socio-economic losses caused by power disturbances and ultimately support a future digital society.

The harmonic probability distribution of power grid with multi-harmonic sources

In this paper, each harmonic value of harmonic sources is obtained in the process of the measured data obtained from power quality monitoring system first. And then, probability distribution of harmonic amplitude are achieved based on maximum entropy method. Considering that the phase distribution is associated with amplitude, a method of dividing amplitude into three parts by expectation and standard deviation is proposed and the phase probability distribution of the corresponding part can be obtained. For non-measured point, a probability distribution method using the aforementioned calculated probability distribution and the mutual impedance between non-measured point and harmonic source is proposed. Finally, fitting error parameters are used to evaluate the fitting effect of probability distribution which prove the feasibility of the method.

Research on an improved matrix pencil method for phasor extraction

Power system monitoring, protection and modal identification typically rely on the accuracy of phasor parameters, thus quickly and accurately extracting phasor is crucial. Based on matrix pencil (MP) method, this paper proposes a fast matrix pencil (FMP) method to estimate parameters of electrical signals. A square matrix was formed by two time domain coupling matrices of input sampling signal. Its eigenvalues represent the changing information of fundamental and harmonic phasors, as well as the pole information of all components. Under the premise of initial phasors, the concerned phasors can be obtained by solving the square matrix eigenvalues. In order to reduce computation, this paper uses the Krylov subspace of square matrix to construct a phasor minimal polynomial, which can be used to quickly calculate concerned eigenvalues of specific components. Theoretical analysis and simulation results demonstrate that the FMP method significantly reduces the calculation workload and has a simple way to achieve, compared to the MP method. It is immune to the effect of non-target components and frequency offset, also keeps a high accuracy with short data window.

Research on the switching method of bank of shunt capacitors in harmonic conditions

A switching method of capacitor banks with different reactance rates in harmonic case is proposed. The proposed method is combined with voltage and reactive power control (VQC). The total switching numbers of capacitor banks are determined by VQC, however, which branches should be switched is decided by harmonic conditions. The substation harmonics are divided into four cases according to the severity of the harmonics in this paper, and appropriate configuration rules of capacitor banks in different harmonic cases are studied combined with the harmonic current compensation analysis supposing the corresponding capacitor banks are switched. This paper presents strategies of capacitor banks switching in various harmonic cases, and the optimal combination of capacitor banks with different reactance rates in different harmonic cases is given with the characteristics of better harmonic compensation as well as voltage and reactive power control. Finally, the validity of the given method is tested through simulation and analysis.