Power transformer energization problems and modeling for calculation of inrush currents

Transformer Energization Problems

This study presents the development and validation of a versatile transformer model specifically designed for analyzing low and medium frequency transient phenomena. An analysis is conducted on the inrush current phenomenon that occurs during the energization of a power transformer with particular focus.

The primary obstacle is to construct a universal model that is applicable to a broad spectrum of investigations, while depending on a restricted quantity of readily accessible input data. The creation of such engineering model is the ultimate objective of the research.

The power transformer is a crucial element in power systems because, apart from transmission lines, it experiences the highest level of exposure to electrical transients among all devices. The models employed to forecast its temporary actions are occasionally insufficient due to insufficient data/measurements and understanding.

Noteworthy situations include inrush currents, switching and lightning impulse strains, generated overvoltages, and harmonics.

An effective method to gain a more profound comprehension of the inrush current phenomena is by doing comprehensive laboratory measurements on distribution transformers and field measurements on power transformers.

Experimental data are quite advantageous in validating the established model.

Figure 1 – Qualitative representation of the inrush current phenomenon and the effect of the residual flux

Research inquiries:

1. Parameter estimation is intricate and lacks complete standardization. It can introduce errors both in the measurement of parameters and in their subsequent processing.
2. The significance of accurately representing the core is often undervalued when saturation is a worry. When considering a transformer model, it is important to consider factors such as core structure, behavior under high saturation, and residual flux initialization.

It should be noted that manually initializing the residual fluxes in a transformer model is a complex task. Comprehending the de-energization transient is crucial for accurately estimating the residual flux values.

Key contributions:

The objective is to create a comprehensive transformer model that can be used for analyzing transitory phenomena.

1. A method for estimating parameters that is backed by sensitivity studies.
2. Rigorous measurements and simulations of ringdown and inrush transients have uncovered new patterns in the initial peak of the inrush current.
3. Possible contribution to the development of novel mitigating techniques.
4. Measurements conducted on three high-capacity power transformers with several megavolt-amperes (MVA).

Figure 2 – Laboratory layout

Scope of Work

It is necessary to develop calculation models in order to forecast electromagnetic pressures on transformers. The current simulation tools commonly employ a single-phase equivalent model, which fails to adequately depict the interconnection between phases resulting from the iron core’s structure and construction. Enhancements should be made to the depiction of hysteresis, core losses, and remanence.

The primary issue lies in the insufficiency of input data, as the conventional test report approach frequently fails to produce a precise model.

The aim of this project is to conduct an in-depth analysis of power transformers and create a transformer model specifically designed for transient simulation investigations. A transformer is a complex electrical apparatus, and accurately representing it in a model is a difficult task. The emphasis is on transformer modeling starting from power frequencies and extending up to just below the first resonant frequency, often in the kilohertz range.

The primary difficulty in transformer modeling lies in accurately representing the non-linear core. A topologically accurate model is necessary to forecast the saturation phenomenon in every region of the core. We will focus specifically on saturation, core losses, and hysteresis loops. Another crucial attribute of the model is its capacity for flux initialization. Hence, a sophisticated nonlinear inductor model is necessary.

Test report data serves as the definitive record of a transformer’s characteristics and is typically the sole source of information. The utilization of readily accessible input data sets a constraint on the level of intricacy that a model can achieve.

Nevertheless, a model derived mostly from this data will be a rather broad model compared to a design-based model (tailored for a single transformer only).

Figure 3 – Trigger signals and systematic variation of the switching times

Enhancing the low frequency/electromagnetic transformer model will often lead to improved accuracy in calculating inrush currents and switching transients. Enhanced understanding of network transients has the potential to enhance power quality and mitigate the likelihood of breakdowns. The initiative will provide significant advantages for both end users and makers of materials and transformers.

From the perspective of utilities, transformer manufacturers, and service providers, this work has the following areas of significance:

1. The presence of precise and dependable simulation tools.
2. The adjustment of protective relays to prevent them from tripping when a transformer is being powered on, in order to minimize the inrush current and maintain optimal power quality.
3. A method to more precisely establish guidelines for the synchronized switching of transformers.
4. Identifying circumstances that may lead to temporary overvoltages. The examination of the origins of failures and the methods to prevent their recurrence.
5. The mitigation of transformer stress conditions.

Outline of the Study

This document primarily aims to offer comprehensive background information and up-to-date expertise on the key elements of transformer modeling. The primary contributions and findings are documented in papers within academic journals and conference proceedings.

The initial Section 1 provides an overview of the reasons and the background of this study.

Section 2 is the main component of the text and is separated into three primary sections: an analysis of low-frequency transformer modeling, background information on inrush current, and an overview of modeling a non-linear hysteretic inductor.

Section 3 provides a comprehensive description of the laboratory and the arrangement of the test. The results of the systematic measurements of energization and de-energization conducted on two distribution transformers are displayed. Furthermore, a limited number of field measurements conducted on big power transformers are showcased.

Section 4 explores many subjects that were prompted by this study and could serve as motivation for future research endeavors.

Specifically, it addresses three main aspects: 1) the creation of the model, 2) the extension of the leakage reactance representation to various winding configurations and quantities, and 3) potential enhancements to the Jiles-Atherton model for transformer modeling.

Chapter 5 encompasses the primary findings of this study and proposes areas of investigation for future research.

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