Noninvasive Measurement of the network impedance for power quality assessment

Posted by Harald Weidl
On 16. August 2024
0 Comments
450kHz, Faults, GIA, Grid Impedance, Power Grid, Research, Supraharmonics

Abstract

Due to a continuous increase in DC loads with power supplies, frequency converters for power and speed control or other non-linear loads, the harmonic levels in a network can increase to such an extent that a disturbance or an impermissible stress on other components occurs. Already in the planning phase it is therefore useful to be able to assess whether a new plant will have an impact on the power quality. This increases the need to monitor the power quality in more detail and, if necessary, to take countermeasures. An important parameter in this context, especially for the design of new plants, is the frequency-dependent network impedance. This paper describes on the one hand a method for a non-invasive meas-urement of the network impedance, which can be integrated in a simple way into the assessment of the power quality. The method is based on the evaluation of transients in the network, which excite the frequency spectrum. As a solution for low voltage, an excitation unit was developed, which ensures the generation of suitable excitations in the network. For measurements in medium voltage, the high voltage and the extra high voltage level transient events in the grid (e.g. the energization of a power transformer or the switching of an overhead line) are used for the excitation of the frequency spectrum. On the other hand, this paper illustrates by a case study the application as well as the benefit of the measurement of network impedances in the clarification of power quality issues.

Introduction

The electrical energy supply from low voltage to extra-high voltage is currently undergoing a series of changes. In both industrial and public grids, a large number of power-electronic energy converters are increasingly being used for the efficient use of electrical energy – from switched-mode power supplies and energy-saving lamps in the watt range to roller drives and electrolysis in the megawatt range to high-voltage direct current transmission (HVDC) in the gigawatt range. DC grids are also being considered as a supplement to traditional AC grids, as outlined in a position paper by the Austrian Association for Electrical Engineering [1].
In the low-voltage sector, the number of PV systems in particular, as well as the number of charging stations and heat pumps, is expected to increase steadily. In the medium voltage range, the feed-in from renewable energies (wind turbines and PV systems) is also expected to increase, as the current version of a study shows [2].

These changes are strongly related to an increase in power electronic systems (PES) in the grid, which can lead to an impairment of the power quality. Accordingly, a continuous change in the power quality situation in the grid is to be expected.
It therefore makes sense to be able to assess the extent to which grid repercussions can occur both in the planning phase of a new system and in existing systems. The IEC 61000-3-X series of standards defines limit values for the interference emission of devices and systems in order to ensure that the operation of a device or system does not have any impact on the grid. The aim of defining these limit values is to ensure that the total interference emission of an installation is always below a certain compatibility level so that there is a sufficient distance between the total interference emission and the interference immunity of the devices/systems (see Figure 1).