No-load Noise Research Articles

New method to optimize no-load noise of power transformers based on core design & transformer operating conditions

This paper introduces a new algorithm to calculate and optimize no-load noise (sound pressure) of power transformers, and to identify iron sheet parameters. The calculation consists of two steps: the 1st step consists in calculating an initial sound pressure level (A-Evaluation) which has approx. 70 % accuracy within a tolerance interval of ±2dB (A). The 2nd step consists in estimating the expected deviation from the initial calculation to reach 90 % accuracy in the final results. This deviation could be due to material handling, quality tolerance, core manufacturing, etc. The optimization process consists of two parts: the 1st part takes place before choosing a certain iron sheet for calculation to identify the sheet parameters required for computational accuracy (“Sheet Optimization”). The 2nd part consists in considering a core design with an undesirable sound pressure level in order to reduce it to an acceptable limit. This part takes into account the other limitations such as no-load losses and transformer dimensions (“Design Optimization”). For new iron sheets in the market, there is no measurement history to rely on. However, the algorithm is also capable of identifying the sheet parameters for calculation based on the available algorithm data base and the magnetostriction measurements of the iron sheet manufacturer.

Direct current compensation – field experience under service conditions

Modern grain oriented core steel used in power transformers has a very high magnetic conductivity. This advanced material makes the transformer susceptible even for small direct current (DC) in the power grid. Already minor DC increases the no-load noise and no-load losses of the transformer considerably. This effect is known as half-cycle saturation. In order to overcome these parasitic DC an active compensation method called “DC compensation” (DCC) was recently developed by Siemens [1]. The question about the origin of the DC is not fully answered yet. However the following sources have been already identified: power electronics, renewable power generation (wind, solar), HVDC transmission lines and DC operated railroad or subway systems. The parasitic direct currents can flow over the power lines to ground or asymmetrically in the power line phases only. In this paper field data, a four-month DC load profile, of single-phase core type transformers, equipped with active DC compensation, are shown. The discussed unit, a bank of three single-phase autotransformers, is in service mainly exposed to DC flowing from the overhead lines through the windings to the common neutral. DC magnitude varies from 0.05 A to about 0.2 A DC per phase throughout the day. From factory tests we know that only 0.2 A DC causes a noise increase of 5.6 dB(A) compared to the noise level without any DC compensation. This might cause troubles at the substation when noise has to be below a guaranteed level. Data analysis of the field data shows that the DC throughout the day follows a clear profile with its highest level during midnight and lunch time. This might indicate a correlation to the load and / or switching operations in the grid to adjust to the actual needed load. However, the DC compensation equipment fully eliminates the direct flux in the core and thus the DC caused increase in noise.

Prediction formula development of power transformer no-load noise

Power transformer noise is one of the contributors to noise pollution in the environment. The dominant component of power transformer noise is no-load noise, which is a noise that appears when a power transformer is connected to a voltage source and is not connected to other electrical loads. The amount of no-load noise must be predicted at the design stage so that the noise does not exceed consumers’ standards or criteria. In this study, the prediction of no-load noise was conducted involving rated power, magnetic flux density, and geometry factor. Based on the measurement results from industrial testing and the multiple linear regression method, no-load noise prediction formula results with better accuracy are obtained.

Magnetostriction of Electrical Steel and Its Relation to the No-Load Noise of Power Transformers

This paper analyzes the relationship between the experimentally determined magnetostriction of grain-oriented electrical steel (GOES) coils and the no-load noise of power transformers (PT). The main source of a PT's no-load noise is the magnetostrictive vibration of the magnetic core built with stacked GOES sheets. With an in-house measurement system, in which steel coils with a mass of up to 6000 kg is measured, differences in the GOES materials’ magnetostriction were identified. In this paper, we analyze the flux dependent behavior and harmonic composition of the magnetostrictive properties of different steel coils. Based on the identified differences in the steel coils’ magnetostriction, two experimental studies with PTs are provided showing exemplary results. The first study shows initial findings of the relationship between the magnetostriction of the deployed steel coils and the emitted no-load noise of model transformers. In a second study, the aforementioned investigations were extended with measurements of large power transformers. Both studies cover a correlation and performance analysis including comparisons of the noise-related GOES certificate value B8 (also known as B800 value).

Experimental Determination of Specific Power Losses and Magnetostriction Strain of Grain-Oriented Electrical Steel Coils

This paper presents a new method for measuring the specific losses and magnetostriction of grain-oriented electrical steel coils of up to 6000 kg. Since large power transformer (LPT) core manufacturers are mainly supplied with entire steel coils, a fast incoming goods inspection is desirable. The proposed method identifies the specific losses of the entire steel coil. In addition, we measured the magnetostriction of the coiled material, which in turn enabled us to classify the main property for LPT no-load noise. Furthermore, we show the structure and setup of the developed coil measurement system. We provide an explanation of the measurement technique and identify its requirements. Exemplary results of specific power losses and magnetostriction of steel coils are provided, and the reproducibility of the results is shown.

Magnetostriction of Transformer Core Steel Considering Rotational Magnetization

As it is well known, the power loss of transformer core steel depends on a series of parameters which vary in the final core in complex ways. The aim of the present study was to investigate the corresponding dependencies for the second key characteristic, i.e., magnetostriction (MS), as the most significant source of no-load noise. The MS-performance of core material was investigated by means of a rotational single sheet tester (RSST). Compared to loss, the peak-to-peak MS in rolling direction (RD; as the direction of strongest strain) shows similar increases with increases of both induction and axis ratio a. On the other hand, the shape of induction pattern B (t) proves to be rather insignificant, MS being similar for elliptic and rhombic magnetization. While increased dynamics of the pattern yields rising eddy current loss, MS remains unaffected. However, the harmonics show increases as being of relevance for audible noise. While mechanical tension in RD yields slight decreases of MS for alternating magnetization, MS for rotational magnetization increases for both tension and compression. As in the case of loss, MS shows significant increases for DC-bias which correlates with reports of increased noise. The study also included local measurements on a 2-limb, 1-phase model core and on a 3-limb, 3-phase core. MS remains in the order of 0.5 ppm for the 1-phase core, except from its corners. On the other hand, T-joint regions of the 3-phase core show values up to the order of 6 ppm, in accordance to RSST-results. Moderate DC magnetization, as being possible in practice as a long term phenomenon, yields distinct increase of MS intensity and MS harmonics with strong regional differences.