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Ferroresonance in single-phase, line-to-line connected transformers in an ungrounded distribution system with delta-connected capacitors is possible but has not been reported in recent literature. In this paper, a high voltage event that actually occurred in an ungrounded distribution network with multiple distribution transformers has been simulated for lessons learned. The hypothesis was that ferroresonance was the cause of the overvoltage event in the network after a single-phase event led to sustained voltages of 1.45 p.u. Simulations were performed and ferroresonance was found to be the possible cause for the overvoltages. One of the prevention solutions found to avoid the overvoltages was to balance the loads on the three phases. The increased deployment of CVR/VVO strategies leads to circuit configurations similar to that studied in this paper and could lead to an increased likelihood of ferroresonance, if it is not fully understood and addressed. The impact of additional single-phase solar inverters on the low voltage side of the transformers was also studied. The results obtained show that the effective loading of the transformers, which is the difference in the actual load connected and the output from the DER, proved to be the deciding factor for initiation of ferroresonance in such networks. • Overvoltage event observed on a utility's ungrounded distribution system. • Ferroresonance was the cause based on electromagnetic transient simulations. • Distributed Energy Resources (DER) added to study impact on ferroresonance. • With DER connected, overvoltage observed at higher values of transformer loading. • Phase balancing and capacitor bank switching avoided overvoltage event. [ABSTRACT FROM AUTHOR]

The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation. [ABSTRACT FROM PUBLISHER]

Current-limiting reactors are placed in series with capacitor banks to limit the rate of rise of current to the values specified in the circuit breaker (CB) standards. But this arrangement has created capacitor bank failures when attempting to clear faults in between the reactor and the capacitor bank. After detailed analyses of failures, solutions have been proposed by researchers: 1) Add a surge capacitor to ground on the capacitor bank side of the breaker and 2) add a surge capacitor across the reactor. These surge capacitors are sized based on the stray capacitances of the bus, the reactor, the circuit breaker, and on the maximum-available fault current at the substation. This paper presents a simplified means of sizing the surge capacitors for method 2), based only on the CB's interrupting current rating and reactor size. This eliminates the need for and uncertainty of stray capacitance values. Also, the design does not need to be revisited when grid enhancements increase the available fault current at a substation. A standard surge protection package, which can also be applied to existing installations, is proposed. This new approach has been verified with studies using Electromagnetic Transients Program/Alternative Transients Program. [ABSTRACT FROM PUBLISHER]

Abstract: The paper documents a new transformer model in ATPDraw called XFMR. This model handles three-phase transformers with two or three windings. Autotransformers and all Wye and Delta couplings are supported. The model includes an inverse inductance matrix for the leakage description, optional frequency dependent winding resistance, capacitive coupling, and a topologically correct core model (3- and 5-legged) with individual saturation and losses in legs and yokes. Three different sources of parameters are supported; typical values, standard test reports, and design information. The hybrid model XFMR is compared to the UMEC model in PSCAD showing good agreement at rated, stationary operation, but considerable differences in transient situations. Both models need further benchmarking and development to reproduce all switching transient behaviors properly. [Copyright &y& Elsevier]

The topological structure and basic approaches for parameter estimation for a new hybrid transformer model are presented in Part I of this two-paper set. Part II deals with the model benchmarking and also discusses additional methods for parameter estimation based on laboratory measurements. The simulation results confirm the validity of the model for the low- and medium-frequency range. [ABSTRACT FROM AUTHOR]

A new topologically correct hybrid transformer model is developed for low and midfrequency transient simulations. Power transformers have a conceptually simple design, but behaviors can be very complex. The selection of the most suitable representation for a given behavior depends on the type of transformer to be simulated, the frequency range, and other factors, such as the internal design of the transformer and available parameters or design data. Here, a modular model suitable for frequencies up to 3–5 kHz is developed, utilizing a duality-based lumped-parameter saturable core, matrix descriptions of leakage and capacitive effects, and frequency-dependent coil resistance. Implementation and testing of this model was done for 15-kVA 208Δ-120Y 3-legged and 150-kVA 12,470Y-208Y 5-legged transformers. The basis and development of the model is presented, along with a discussion of necessary parameters and the approaches for obtaining them. [ABSTRACT FROM AUTHOR]

One of the weakest components of modem transient simulation software is the transformer model. Many opportunities exist to improve the simulation of its complicated behaviors, which include magnetic saturation of the core, frequency-dependency, capacitive coupling, and topologically correctness of core and coil structure. This paper presents a review of transformer models for simulation of low- and mid-frequency transients. Salient points of key references are presented and discussed in order to give an accessible overview of development, implementation and limitations of the most useful models proposed to date. Techniques for representation of the most important behaviors are examined, with the intent of providing the needed foundation for continued development and improvement of transformer models. [ABSTRACT FROM AUTHOR]

Over the years, Michigan Tech has had two different philosophies for teaching design in the power area initially, we integrated design into topical courses, teaching the material the students would need to complete the design. Presently, our students complete year-long, industry-sponsored projects. We will, give details on the implementation of the two approaches and compare and contrast the outcomes. Both techniques for teaching design have their advantages and disadvantages. [ABSTRACT FROM AUTHOR]

This paper presents a simple modulation index-dependent Thévenin equivalent circuit model of the voltage-source converters (VSCs) and the associated anti-parallel diode rectifier. The model derivation uses orthogonal transformation and a transformer analogy to transfer impedances to the dc side of the converter. The paper proposes a switched equivalent representation of a three-phase diode rectifier formed due to the antiparallel diodes when VSC switching is inhibited. The proposed model is implemented in Alternate Transients Program–Electromagnetic Transients Program and simulation results are compared with the switched model. The proposed model is shown to under predict the fault currents for pole-ground (PG) faults and to provide a nearly exact match for pole-pole faults. The rectifier equivalent provides increased accuracy in the transient region with the inclusion of the source inductance. A multiterminal dc (MTDC) system model is used to study effects of aggregation which gives increased accuracy of the equivalent circuit. It is shown that the MTDC system simulation with the proposed Thévenin equivalent is about 3 times faster than full-switched circuit simulation for a time step of 5 \mus. The MTDC system simulation with the Thévenin equivalent circuit is still accurate and 126 times faster when the time step is increased to 750 \mus. [ABSTRACT FROM AUTHOR]

This paper presents a power swing detection algorithm in multi-machine power systems, which uses wide-area synchrophasor measurements and is based on Zubov’s stability boundaries. Zubov’s boundary method, an approximation of the famous Lyapunov function, is very common for numerical analyses of power system stability. This work extends the concept of Zubov’s approximation boundaries to online protection applications. First, the proposed protection algorithm constructs a series of Zubov’s stability boundaries on power angle–frequency ($\delta$ – $\omega$) plane for generators based on real-time measurements from phasor measurement units/merging units. Then, it tracks the trajectory of each generator in the $\delta$ – $\omega$ plane after the disturbance to differentiate out-of-step (OOS) conditions from stable swings. The proposed approach is suitable for online applications, which is a significant improvement over the existing methods for swing detection since it does not require offline studies and network reduction. Effectiveness of the proposed method is shown using a single-machine infinite bus and New-England 10-machine 39-bus systems. The proposed method is also implemented in a hardware-in-the-loop setup with a customized IEC 61850-based OOS relay, and performance is compared with a commercially available relay. [ABSTRACT FROM AUTHOR]

This paper investigates the voltage distortion phenomenon during no-load testing of transformers, its influence on no-load loss calculations, and a novel measurement technique that is unaffected by harmonics for obtaining transformer magnetization curves. Owing to economic reasons, no-load loss correction for distorted test waveforms has been addressed to some extent in existing standards, while not much has been done on the aspect of calculation of magnetization characteristics. This is important for parameter estimation and transformer modeling in power system transient simulations. Results from two test cases are presented, one is based on factory testing with 290-MVA, three-phase, three-legged power transformer and second is a more comprehensive analysis using a laboratory test setup with a 22-kVA single-phase transformer. Application of loss correction equations and limits based on existing testing standards was evaluated and found to result in overcompensation, while calculation of magnetization curves based on existing methods resulted in error up to 20%. Whereas, application of proposed measurement technique based on voltage “Zero-Crossing” detection is shown to result in negligible error. The proposed measurement technique uses the same input signals as in a standard no-load test procedure. Hence, it can be easily implemented in parallel with existing instrumentation. [ABSTRACT FROM AUTHOR]

In this paper, the concept of an all-dc wind park with series-connected turbines is investigated as an alternative to the classical ac parallel or radial wind park. This paper presents a literature overview of all-dc wind park concepts with series connection. A three-phase conversion system with permanent magnet machine, ac–ac converter, high-frequency transformer, and diode bridge rectifier is suggested in this paper for the series connection of dc turbines. The dc series park with the suggested conversion system is compared in terms of losses, cost, and reliability to the state-of-the-art park configuration which is the ac radial park with HVDC transmission. It is found that the dc series park becomes comparable with the ac radial design for high ratings of the dc turbines. Furthermore, the comparison shows that emphasis must be put on reducing the losses in the conversion system of the dc turbine and, particularly, the ac–ac converter. Therefore, the efficiency of the ac–ac converter is compared for three different topologies: the direct matrix converter, the indirect matrix converter, and the conventional back-to-back converter. The direct matrix converter is found to be the most efficient, suitable for the suggested conversion system. [ABSTRACT FROM PUBLISHER]