Method for determining customer contribution to harmonic variations in a large power network
A method for identifying the source of harmonic emission and for determining its contribution to the harmonic distortion at different points on a large power supply grid is presented in this paper. The proposed method is based on vector analysis of the harmonic currents, measured synchronously at multiple points on the electrical grid. This approach allows real time locating of the responsible customer (harmonic cause) for a given harmonic current disturbance and determining its contribution to the harmonic voltage and current variations at all measured points (harmonic effect). A physical model, representing a power network supplying three non-linear loads, has been used to illustrate the performances of this method. The presented method can be used to analyze the harmonic current and voltage variations in a large power network in real time, thus assisting the electrical power utility to rapidly identify the harmonic causes and their effect and to better understand different problems associated with the harmonics.
A Piecewise Bound Constrained Optimization for Harmonic Responsibilities Assessment under Utility Harmonic Impedance Changes
Considering the effect of the utility harmonic impedance variations on harmonic responsibility, a method based on piecewise bound constrained optimization is proposed in this paper to evaluate the load harmonic responsibilities. The wavelet packet transform is employed to determine the change times of the utility harmonic impedances. The harmonic monitoring data is divided into several segments where the utility harmonic impedances are considered as constants. Then, the problem of harmonic responsibility assessment under utility harmonic impedance changes are settled by the piecewise bound constrained optimization model. Furthermore, the interior point, the sequential quadratic programming and the active set algorithm are respectively adopted to calculate all the instantaneous harmonic responsibilities of harmonic loads. Finally, the weighted summation is used to calculate the total harmonic responsibility. To demonstrate the validity, simulation tests are carried out on an experimental circuit and the IEEE 13-bus distribution system.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.