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This paper presents an experimental study of the magnetic effects of steel tanks on different three-phase transformers. Removable steel tanks are designed and built for transformers with distinct sizes and core topologies. Corresponding tanks of nonmagnetic materials are also built to study the effects of tank material. Zero-sequence tests, open-circuit (no-load) tests, ferroresonance, geomagnetically induced current (GIC), and energization (inrush current) tests are performed on the transformers with and without tanks. The results indicate that steel tanks have a significant effect on the zero-sequence impedance, ferroresonance performance, GIC, and no-load losses in steady state, while the tank has no appreciable effect on inrush currents.

This two-part paper presents a method to mitigate half-cycle saturation of transformers caused by monopolar operation of neighboring HVDC transmission lines while keeping the ability to detect the zero-sequence fault currents (ZSFC) when ground faults occur. Part I of this paper has presented the mitigation principles and device design of the proposed neutral current blocking switch. In Part II, the performance of the proposed method to permit the circulation of zero-sequence current is investigated. An operation strategy is proposed that simultaneously allows the mitigation of half-cycle saturation and the detection of ZSFC. Simulations on a widely-used 500 kV system show that the proposed mitigation technique, using a sub-synchronous switching frequency (no higher than 30 Hz), can effectively mitigate the half-cycle saturation while allowing the circulation of ZSFC. The novel mitigation method exploits the characteristic differences between half-cycle saturation (dc) and asymmetric faults (ac). The method provides an implementable solution to the dc-bias phenomenon because it delivers concurrently dc-bias mitigation, minimal impact on ground fault detection, and no switching stresses on the power electronic switches. The proposed technique can also be applied to the mitigation of geomagnetically induced currents.

When HVDC transmission systems operate in the ground return mode, transformers in adjacent ac systems frequently suffer half-cycle saturation. In this two-part paper, an effective mitigation method using power electronic switches is proposed that does not compromise the ability of the protection system to detect fault currents. In Part I, a study is conducted to find the physical performance of the half-cycle saturation and give a technical solution to the inrush-like half-cycle saturated currents. Power electronic switches, opening the neutral connection to ground, are utilized to mitigate the saturation without compromising the reference ground of the transformer. The determination of the switch parameters, switching frequency, and duty cycle is performed. Details of the mitigation transient are studied by comparing simulations and laboratory experiments to validate the efficacy of the proposed power electronic switch for the mitigation of half-cycle saturated currents. In Part II, the influence of the proposed switch on the detection of ground faults is investigated. Proper parameter and operation strategies are selected to effectively detect the ground faults and ensure the fast response of protective relays. The techniques proposed are applicable to mitigate geomagnetically induced current.

This paper presents a novel circuit model for eddy-current effects for multi-winding transformers. It presents the theoretical framework to model eddy currents applicable to layer or disk windings with n-layers for two-dimensional (2-D) transformer arrangements. The new white-box model is physical, dual, and leakage reversible. Additionally, it does not have mutually coupled elements or negative inductors. The paper presents a complete circuit for transformer windings (including capacitances) and core. The model accurately predicts the variation of resistance (dc component, skin, and proximity effects) and leakage inductance of windings from dc to hundreds of kilohertz. Being derived from the principle of duality, the circuit elements can be physically related one-to-one to the distribution of flux and current in the winding geometry. A practical winding discretization, based on field penetration depth for the given frequency, allows estimating the parameters using very simple formulae requiring only geometrical information and properties of materials. Model verification is done using finite element simulations. It is shown that the circuit model matches very well with 2-D FEM simulations. The circuit can be easily implemented in any circuit simulation software like EMTP-RV, PSCAD, ATP, etc., by simply dragging and dropping elements.

In this paper, an extension to the multiwinding BCTRAN model is proposed for the study of low-frequency saturating transients in single-phase transformers. The conventional experimentally obtained BCTRAN (winding leakage) susceptance matrix is retrofitted with a previously missing experimentally obtained nonlinear (core magnetizing) model. The assembled model gives accurate results for transients in all terminals. The model parameters can be acquired from information available in the nameplate plus terminal tests without the need of detailed transformer design or construction information. Only needed are standard no-load and impedance tests plus a newly introduced saturation test. Very simple formulas are proposed to compute the parameters. Illustrative examples on how the parameters are computed are given for three transformers rated 1.8, 5, and 75 kVA. For model validation, laboratory inrush currents tests are carried out and compared with simulations using the EMTP-RV. The results show that the simulations and experimental results match very closely giving great confidence in the model correctness and parameter estimation method.

This paper presents a novel online diagnostics method capable of detecting winding deformations in two-winding single-phase transformers. The main idea is to identify changes in the short-circuit impedance. The combination of 3-D Lissajous curve methods with a Butterworth low-pass filter allows for the accurate determination of winding deformation of large power transformers in real time. The method is very robust and capable of detecting deformations at the early stage even when the measurements are noisy. Only information already available to the differential protection relay is needed. The proposed diagnostics method has been validated with circuit and finite-element simulations plus a lab experiment. The results show that the proposed online diagnostics technique has the ability to identify winding deformation problems under severe conditions, such as nonsinusoidal input, nonlinear loading, and measurement noise. Under ideal conditions (no signal noise), the inductive identification error of the proposed online diagnostics method identifies the parameters with less than 0.09% error. When accepting a measurement noise of 1%, the error on the identification of inductance is less than 0.13%.

The purpose of core demagnetization is twofold: 1) to reduce the inrush currents when transformers are energized; and 2) to make sure that the frequency-response analysis (FRA) tests are consistent to avoid false diagnoses of damage during transportation. The significance of demagnetizing is presented on field measurements of an 80 MVA unit with FRA measurements. A new demagnetizer device with an alternating polarity dc voltage source is prototyped. Experimental verification of this prototype is presented for the demagnetization of transformers. A nearly complete demagnetization was observed in the laboratory for a small single-phase isolation transformer. The method proposed in this paper is applied to three-phase transformers with different core configurations and connections. Topologically correct modeling and numerical simulations confirm the full demagnetization of all branches of three-phase (three- and five-limb) transformer cores. Inrush current measurements and FRA plots before and after demagnetization confirm the effectiveness of the process.

This paper presents a physically consistent dual model applicable to single-phase two-winding transformers for the calculation of low-frequency transients. First, the topology of a dual electrical equivalent circuit is obtained from the direct application of the principle of duality. Then, the model parameters are computed considering the variations of the transformer electromagnetic behavior under various operating conditions. Current modeling techniques use different topological models to represent diverse transient situations. The reversible model proposed in this paper unifies the terminal and topological equivalent circuits. The model remains invariable for all low-frequency transients including deep saturation conditions driven from any of the two windings. The proposed model is tested with a single-phase transformer for the calculation of magnetizing inrush currents, series ferroresonance, and geomagnetic-induced currents (GIC). The electromagnetic transient response of the model is compared to the π model and to laboratory measurements for validation.