Francisco de Leon

Experimental Determination of the ZIP Coefficients for Modern Residential, Commercial, and Industrial Loads

This paper presents the experimental determination of the ZIP coefficients model to represent (static) modern loads under varying voltage conditions. ZIP are the coefficients of a load model comprised of constant impedance Z, constant current I, and constant power P loads. A ZIP coefficient load model is used to represent power consumed by a load as a function of voltage. A series of surveys was performed on typical residential, commercial, and industrial customers in New York City. Household appliances and industrial equipment found in the different locations were tested in the laboratory by varying the voltage from 1.1-p.u. voltage to 0 and back to 1.1 pu in steps of 3 V to obtain the individual P- V, Q- V, and I- V characteristics. Customer load tables were built using seasonal factors and duty cycles to form weighted contributions for each device in every customer class. The loads found in several residential classes were assembled and tested in the lab. It was found that modern appliances behave quite differently than older appliances even from only 10 years back. Models of the different customer classes were validated against actual recordings of load variations under voltage reduction.

A Robust Multiphase Power Flow for General Distribution Networks

This paper presents a sweep-based three-phase power flow method for solving general distribution networks that can be heavily meshed and include transformers around the meshes/loops. A load-stepping technique is proposed for solving common convergence problems of sweep-based load-flow solvers when dealing with overloaded radial sections. The proposed power-flow algorithm is based on the iterative solution of radial subsystems assembled together with the mesh equations to comply with Kirchhoff equations. The proposed method is robust and efficient for the solution of heavily loaded systems. Examples are presented for illustration.

Field-Validated Load Model for the Analysis of CVR in Distribution Secondary Networks: Energy Conservation

This paper presents a field-validated load model for the calculation of the energy conservation gains due to conservation voltage reduction (CVR) in highly meshed secondary networks. Several networks in New York City are modeled in detail. A time resolution of one hour is used to compute the energy savings in a year. A total of 8760 power flow runs per year for voltage reductions of 0%, 2.25%, 4%, 6%, and 8% from the normal schedule are computed. An equivalent ZIP model is obtained for the network for active and reactive powers. The most important finding is that voltage reductions of up to 4% can be safely implemented in the majority of the New York City networks, without the need of investments in infrastructure. The networks under analysis show CVR factors between 0.5 and 1 for active power and between 1.2 and 2 for reactive power, leading to the conclusion that the implementation of CVR will provide energy and economic savings for the utility and the customer.

Leakage Inductance Design of Toroidal Transformers by Sector Winding

Toroidal transformers are commonly used in power electronics applications when the volume or weight of a component is at a premium. There are many applications that require toroidal transformers with a specific leakage inductance value. A transformer with a large (or tuned) leakage inductance can be used to eliminate a (series) filter inductor. In this paper, a procedure to control the leakage inductance of toroidal transformers by leaving unwound sectors in the winding is presented. Also, a simple formula is obtained in this paper that can be used to design transformers with a specific leakage inductance value. The leakage inductance formula is expressed as a function of the number of turns, the geometrical dimensions of the toroidal transformer, such as core internal diameter, external diameter, and height, and the angle of the unwound sector. The formula proposed in this paper has been obtained and validated from laboratory experiments and hundreds of three-dimensional finite element simulations. The techniques described in this paper will find applications in the design of transformers that in addition of providing voltage boosting need to double as filters.

Dual Reversible Transformer Model for the Calculation of Low-Frequency Transients

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.

Accurate Measurement of the Air-Core Inductance of Iron-Core Transformers With a Non-Ideal Low-Power Rectifier

The air-core inductance of power transformers is measured using a nonideal low-power rectifier. Its dc output serves to drive the transformer into deep saturation, and its ripple provides low-amplitude variable excitation. The principal advantage of the proposed method is its simplicity. For validation, the experimental results are compared with 3-D finite-element simulations.

Impulse-Response Analysis of Toroidal Core Distribution Transformers for Dielectric Design

Toroidal transformers are currently used only in low-voltage applications. There is no published experience for toroidal transformer design at distribution-level voltages. This paper explores the lightning impulse response of toroidal distribution transformers in order to obtain a dielectric design able to withstand standardized impulse tests. Three-dimensional finite-element simulations are performed to determine the capacitance matrix on a turn-to-turn basis. Then, a lumped parameter RLC model is applied to predict the transient response of the winding as well as to obtain the potential distribution along the winding and corresponding dielectric stresses. The model computes the impulse potential distribution and the dynamic (interturn and interlayer) dielectric stresses. Different insulation design strategies are proposed by means of electrostatic shielding and variation of the interlayer insulation.

AC Power Theory From Poynting Theorem: Accurate Identification of Instantaneous Power Components in Nonlinear-Switched Circuits

This paper contributes to narrowing the long-standing theoretical gap with power theory (or “power definitions”) for nonlinear ac switching circuits. The true instantaneous energy transformation and storage components of ac circuits are identified from the Poynting Theorem. This paper tackles the problem of power identification from the most general form of energy conservation. Therefore, it is no longer necessary to mathematically “define” powers to fit the engineering solution of a problem. The identification technique does not present problems with physical meaning since it is in full agreement with Maxwells Equations. In this paper, the method is applied to the identification of the power components of single-phase switched circuits. Instantaneous energy is decomposed only into energy transformed (related to active power) and energy stored (related to reactive power). Examples that have caused physical interpretation problems with other power theories are presented for illustration and validation.

Mitigation of Geomagnetically Induced Currents by Neutral Switching

This paper presents a novel mitigation technique to reduce the effects of geomagnetically induced currents (GICs) on high-voltage power systems. The method consists of connecting switching devices at the neutral grounding connection point of transformer banks. Only one transformer bank needs to be grounded through a switch to reduce GIC in a two-terminal system. For multiterminal systems, n-1 switches are necessary and the operation is independent from each other. The switching frequency and the duty cycle are selected from a tradeoff between the effectiveness of the method and the detection of fault currents. Transient simulations on a two-bus 230/500 kV system show that the proposed switching at the neutral successfully mitigates the GIC. This proposed technique opens the door to a new family of GIC mitigation methods.

Transformer Leakage Flux Models for Electromagnetic Transients: Critical Review and Validation of a New Model

This paper presents experimental validation of the coupled leakage inductance transformer model. It is shown that the coupled approach yields the same results as the indefinite admittance matrix method of BCTRAN. A topologically correct three-phase shell-type transformer model is proposed. The connection points between the leakage and magnetizing inductances are properly identified, which makes the new model superior to BCTRAN and the hybrid models, by providing physical consistency. Additionally, experimental verification of a method to calculate the short-circuit inductances is presented. New explanations on the division of leakage flux and on the mathematical equivalence between the T- and ↑-equivalent models are also given.