Study on the Measuring Harmonics Based on Capacitor Voltage Transformer

In high voltage grade power networks, people usually use capacitive voltage transformers (CVT) to measure the harmonic voltage, but the results exist errors. This paper studied the basic structure of CVT, established the equivalent circuit model of CVT, and proposed a method to calculate the harmonic voltage of the grid side by sampling the grounding current signal of CVT. On the basis of that, the paper confirmed the correctness and feasibility of the method through model simulation. Introduction As the traditional impact load continues to develop, for example, power electronics technology has been widely and 110kV, 220kV grid-connected electric furnace steelmaking. The problems of power quality it caused, has obviously affected power system operation and the safety of the user's facilities [1,2]. Among all kinds of power quality problems, the harmonic, which has a wide range of effects, is a main indicator that needs to be monitored in power quality. The harmonic voltage, which is measured by the voltage transformer, is the basic parameter of measurement and analysis in the study of harmonic problems. Among kinds of voltage transformer, “CVT” which is the common short form for “capacitor voltage transformer”, is mainly used in 110kV, 220kV and higher voltage grade grids [3]. Harmonic measurement results are basically derived from CVT in grids, which is applied CVT, with voltage levels of 110kV and above. But the transmission of harmonics by CVT has actually undergone a nonlinear change, in other words, the secondary side harmonic voltage measured by CVT can't reflect the true harmonic characteristics on the high voltage side. It is also clearly stated in GB/T14549-1993 that CVT can’t be used to measure harmonic voltage of power system [4]. If we can design a reasonable and effective measurement scheme, which is use the existing CVT to measure the percentage of harmonic voltage and obtain the true harmonic content in the high-voltage power grid. We will get more and more valuable references helping us solve the problem of monitoring power quality and governing harmonic in electric power system. This paper proposes a research method for harmonic measurement of CVT, based on an in-depth study of the basic structure of CVT. The research provides strong support for the management decision of power grid operation and maintenance department, improving the safety and economy of grid operation. Basic Structure and Equivalent Circuit Model of CVT Basic Structure of CVT The basic structure of the CVT is shown in Figure 1. It consists of a capacitor divider unit and an electromagnetic unit [5]. The capacitor divider unit consists of a high voltage capacitor and a low voltage capacitor. And the electromagnetic unit is composed of a compensating reactor and an intermediate transformer. The grid-side voltage is firstly reduced to medium voltage by the capacitive voltage divider, and then to low voltage signal for measurement, control and relay protection [6]. International Conference on Modeling, Analysis, Simulation Technologies and Applications (MASTA 2019) Copyright © 2019, the Authors. Published by Atlantis Press. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). Advances in Intelligent Systems Research, volume 168